-Fixes to undo redo to avoid crash, closes #24251

-Changed Animation to have a special signal when tracks are changed, to avoid unnecesary track cache rebuilds in AnimationPlayer
-Added missing emit_changed whe modifying keys to Animation
-Changed AnimationPlayer to use the new refcounted connections instead of the previous hacky way to keep references
-Changed AnimationEditor to update the current track when keys are edited
-Fixed bug where undo/redo did not work with AnimationKeyEdit (was not being updated)
-Made sure UndoRedo does not mind deleted objects in undo/redo history, this would corrupt the history or clear it without need.

scene/scene_string_names.cpp

@@ -203,4 +203,6 @@ SceneStringNames::SceneStringNames() {
 	_mesh_changed = StaticCString::create("_mesh_changed");
 
 	parameters_base_path = "parameters/";
+
+	tracks_changed = "tracks_changed";
 }