add builtin icons as a fallback option and as a way to use the editor…

…'s icons

This check makes it so, in case any custom icon finding or opening were to fail, it fallbacks on a builtin icon in the theme with the same name. This way, we can specify an icon that's already in the engine instead of having to provide it. For example, if you wanted your CustomNode icon to be that of a CharacterBody2D, you could specify it as a path with "CharacterBody2D".

editor/editor_node.cpp

@@ -4846,6 +4846,17 @@ Ref<Texture2D> EditorNode::_get_class_or_script_icon(const String &p_class, cons
 		if (theme->has_icon(p_class, EditorStringName(EditorIcons))) {
 			return theme->get_icon(p_class, EditorStringName(EditorIcons));
 		}
+
+		// If there is a path associated with the node but there was no icon there or it was invalid.
+		// Then it takes the basename of the file and, if it's the name of an icon in the theme, it uses that instead as a fallback.
+		// This enables using "Node2D" as a valid path to get the icon without having to copy it and have it inside the project folder.
+		if (GDExtensionManager::get_singleton()->class_has_icon_path(p_class)) {
+			String icon_name = GDExtensionManager::get_singleton()
+			->class_get_icon_path(p_class).get_file().get_basename();
+			if (theme->has_icon(icon_path, EditorStringName(EditorIcons))) {
+				return theme->get_icon(icon_path, EditorStringName(EditorIcons));
+			}
+		}
 
 		if (!p_fallback.is_empty() && theme->has_icon(p_fallback, EditorStringName(EditorIcons))) {
 			return theme->get_icon(p_fallback, EditorStringName(EditorIcons));