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/resources/texture.cpp

@@ -1790,7 +1790,7 @@ void GradientTexture::_queue_update() {
 	}
 
 	update_pending = true;
-	call_deferred("_update");
+	call_deferred(SNAME("_update"));
 }
 
 void GradientTexture::_update() {