[Window] Add unfiltered input handler signal for custom decorations.

scene/scene_string_names.h

@@ -53,6 +53,7 @@ public:
 	const StringName input_event = "input_event";
 	const StringName gui_input = "gui_input";
 	const StringName window_input = "window_input";
+	const StringName nonclient_window_input = "nonclient_window_input";
 
 	const StringName tree_entered = "tree_entered";
 	const StringName tree_exiting = "tree_exiting";