Introduce the concept of global theme contexts

This commit adds the default theme context, which replaces
the need to manually check the project and the default theme
all the time; simplifies related code.

It also adds framework for custom theme contexts, to be used
by the editor. Custom contexts can be attached to any node,
and not necessarily a GUI/Window node. Contexts do no break
theme inheritance and only define which global themes a node
uses as a fallback.

Contexts propagate NOTIFICATION_THEME_CHANGED when one of their
global themes changes. This ensures that global themes act just
like themes assigned to individual nodes and can be previewed
live in the editor.

1 files changed, 11 insertions, 19 deletions

diff --git a/scene/resources/primitive_meshes.cpp b/scene/resources/primitive_meshes.cpp
index a64ae07f05..6d848f5494 100644
--- a/scene/resources/primitive_meshes.cpp
+++ b/scene/resources/primitive_meshes.cpp
@@ -3466,32 +3466,24 @@ Ref<Font> TextMesh::_get_font_or_default() const {
 		return font_override;
 	}
 
-	// Check the project-defined Theme resource.
-	if (ThemeDB::get_singleton()->get_project_theme().is_valid()) {
-		List<StringName> theme_types;
-		ThemeDB::get_singleton()->get_project_theme()->get_type_dependencies(get_class_name(), StringName(), &theme_types);
-
-		for (const StringName &E : theme_types) {
-			if (ThemeDB::get_singleton()->get_project_theme()->has_theme_item(Theme::DATA_TYPE_FONT, "font", E)) {
-				return ThemeDB::get_singleton()->get_project_theme()->get_theme_item(Theme::DATA_TYPE_FONT, "font", E);
-			}
+	StringName theme_name = "font";
+	List<StringName> theme_types;
+	ThemeDB::get_singleton()->get_native_type_dependencies(get_class_name(), &theme_types);
+
+	ThemeContext *global_context = ThemeDB::get_singleton()->get_default_theme_context();
+	for (const Ref<Theme> &theme : global_context->get_themes()) {
+		if (theme.is_null()) {
+			continue;
 		}
-	}
-
-	// Lastly, fall back on the items defined in the default Theme, if they exist.
-	{
-		List<StringName> theme_types;
-		ThemeDB::get_singleton()->get_default_theme()->get_type_dependencies(get_class_name(), StringName(), &theme_types);
 
 		for (const StringName &E : theme_types) {
-			if (ThemeDB::get_singleton()->get_default_theme()->has_theme_item(Theme::DATA_TYPE_FONT, "font", E)) {
-				return ThemeDB::get_singleton()->get_default_theme()->get_theme_item(Theme::DATA_TYPE_FONT, "font", E);
+			if (theme->has_font(theme_name, E)) {
+				return theme->get_font(theme_name, E);
 			}
 		}
 	}
 
-	// If they don't exist, use any type to return the default/empty value.
-	return ThemeDB::get_singleton()->get_default_theme()->get_theme_item(Theme::DATA_TYPE_FONT, "font", StringName());
+	return global_context->get_fallback_theme()->get_font(theme_name, StringName());
 }
 
 void TextMesh::set_font_size(int p_size) {