New and improved IK system for Skeleton2D

This PR and commit adds a new IK system for 2D with the Skeleton2D node
that adds several new IK solvers, a way to control bones in a Skeleton2D
node similar to that in Skeleton3D. It also adds additional changes
and functionality.

This work was sponsored by GSoC 2020 and TwistedTwigleg.

Full list of changes:
* Adds a SkeletonModifier2D resource
  * This resource is the base where all IK code is written and executed
  * Has a function for clamping angles, since it is so commonly used
  * Modifiers are unique when duplicated so it works with instancing
* Adds a SkeletonModifierStack2D resource
  * This resource manages a series of SkeletonModification2Ds
  * This is what the Skeleton2D directly interfaces with to make IK possible
* Adds SkeletonModifier2D resources for LookAt, CCDIK, FABRIK, Jiggle, and TwoBoneIK
  * Each modification is in its own file
  * There is also a SkeletonModifier2D resource that acts as a stack for using multiple stacks together
* Adds a PhysicalBone2D node
  * Works similar to the PhysicalBone3D node, but uses a RigidBody2D node
* Changes to Skeleton2D listed below:
  * Skeleton2D now holds a single SkeletonModificationStack2D for IK
  * Skeleton2D now has a local_pose_override, which overrides the Bone2D position similar to how the overrides work in Skeleton3D
* Changes to Bone2D listed below:
  * The default_length property has been changed to length. Length is the length of the bone to its child bone node
  * New bone_angle property, which is the angle the bone has to its first child bone node
  * Bone2D caches its transform when not modified by IK for IK interpolation purposes
  * Bone2D draws its own editor gizmo, though this is stated to change in the future
* Changes to CanvasItemEditor listed below:
  * Bone2D gizmo drawing code removed
  * The 2D IK code is removed. Now Bone2D is the only bone system for 2D
* Transform2D now has a looking_at function for rotating to face a position
* Two new node notifications: NOTIFICATION_EDITOR_PRE_SAVE and NOTIFICATION_EDITOR_POST_SAVE
  * These notifications only are called in the editor right before and after saving a scene
  * Needed for not saving the IK position when executing IK in the editor
* Documentation for all the changes listed above.

1 files changed, 104 insertions, 0 deletions

diff --git a/doc/classes/SkeletonModification2D.xml b/doc/classes/SkeletonModification2D.xml
new file mode 100644
index 0000000000..8596dac76e
--- /dev/null
+++ b/doc/classes/SkeletonModification2D.xml
@@ -0,0 +1,104 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="SkeletonModification2D" inherits="Resource" version="4.0">
+	<brief_description>
+		A resource that operates on [Bone2D] nodes in a [Skeleton2D].
+	</brief_description>
+	<description>
+		This resource provides an interface that can be expanded so code that operates on [Bone2D] nodes in a [Skeleton2D] can be mixed and matched together to create complex interactions.
+		This is used to provide Godot with a flexible and powerful Inverse Kinematics solution that can be adapted for many different uses.
+	</description>
+	<tutorials>
+	</tutorials>
+	<methods>
+		<method name="_draw_editor_gizmo" qualifiers="virtual">
+			<return type="void">
+			</return>
+			<description>
+				Used for drawing [b]editor-only[/b] modification gizmos. This function will only be called in the Godot editor and can be overriden to draw custom gizmos.
+				[b]Note:[/b] You will need to use the Skeleton2D from [method SkeletonModificationStack2D.get_skeleton] and it's draw functions, as the [SkeletonModification2D] resource cannot draw on its own.
+			</description>
+		</method>
+		<method name="_execute" qualifiers="virtual">
+			<return type="void">
+			</return>
+			<argument index="0" name="delta" type="float">
+			</argument>
+			<description>
+				Executes the given modification. This is where the modification performs whatever function it is designed to do.
+			</description>
+		</method>
+		<method name="_setup_modification" qualifiers="virtual">
+			<return type="void">
+			</return>
+			<argument index="0" name="modification_stack" type="SkeletonModificationStack2D">
+			</argument>
+			<description>
+				Called when the modification is setup. This is where the modification performs initialization.
+			</description>
+		</method>
+		<method name="clamp_angle">
+			<return type="float">
+			</return>
+			<argument index="0" name="angle" type="float">
+			</argument>
+			<argument index="1" name="min" type="float">
+			</argument>
+			<argument index="2" name="max" type="float">
+			</argument>
+			<argument index="3" name="invert" type="bool">
+			</argument>
+			<description>
+				Takes a angle and clamps it so it is within the passed-in [code]min[/code] and [code]max[/code] range. [code]invert[/code] will inversely clamp the angle, clamping it to the range outside of the given bounds.
+			</description>
+		</method>
+		<method name="get_editor_draw_gizmo" qualifiers="const">
+			<return type="bool">
+			</return>
+			<description>
+				Returns whether this modification will call [method _draw_editor_gizmo] in the Godot editor to draw modification-specific gizmos.
+			</description>
+		</method>
+		<method name="get_is_setup" qualifiers="const">
+			<return type="bool">
+			</return>
+			<description>
+				Returns whether this modification has been successfully setup or not.
+			</description>
+		</method>
+		<method name="get_modification_stack">
+			<return type="SkeletonModificationStack2D">
+			</return>
+			<description>
+				Returns the [SkeletonModificationStack2D] that this modification is bound to. Through the modification stack, you can access the Skeleton3D the modification is operating on.
+			</description>
+		</method>
+		<method name="set_editor_draw_gizmo">
+			<return type="void">
+			</return>
+			<argument index="0" name="draw_gizmo" type="bool">
+			</argument>
+			<description>
+				Sets whether this modification will call [method _draw_editor_gizmo] in the Godot editor to draw modification-specific gizmos.
+			</description>
+		</method>
+		<method name="set_is_setup">
+			<return type="void">
+			</return>
+			<argument index="0" name="is_setup" type="bool">
+			</argument>
+			<description>
+				Manually allows you to set the setup state of the modification. This function should only rarely be used, as the [SkeletonModificationStack2D] the modification is bound to should handle setting the modification up.
+			</description>
+		</method>
+	</methods>
+	<members>
+		<member name="enabled" type="bool" setter="set_enabled" getter="get_enabled" default="true">
+			If [code]true[/code], the modification's [method _execute] function will be called by the [SkeletonModificationStack2D].
+		</member>
+		<member name="execution_mode" type="int" setter="set_execution_mode" getter="get_execution_mode" default="0">
+			The execution mode for the modification. This tells the modification stack when to execute the modification. Some modifications have settings that are only availible in certain execution modes.
+		</member>
+	</members>
+	<constants>
+	</constants>
+</class>