Optimize StringName usage

* Added a new macro SNAME() that constructs and caches a local stringname.
* Subsequent usages use the cached version.
* Since these use a global static variable, a second refcounter of static usages need to be kept for cleanup time.
* Replaced all theme usages by this new macro.
* Replace all signal emission usages by this new macro.
* Replace all call_deferred usages by this new macro.

This is part of ongoing work to optimize GUI and the editor.

scene/3d/audio_stream_player_3d.cpp

@@ -607,7 +607,7 @@ void AudioStreamPlayer3D::_notification(int p_what) {
 		//stop playing if no longer active
 		if (!active.is_set()) {
 			set_physics_process_internal(false);
-			emit_signal("finished");
+			emit_signal(SNAME("finished"));
 		}
 	}
 }