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.

modules/visual_script/visual_script_nodes.cpp

@@ -2987,7 +2987,7 @@ VisualScriptNodeInstance *VisualScriptCustomNode::instantiate(VisualScriptInstan
 }
 
 void VisualScriptCustomNode::_script_changed() {
-	call_deferred("ports_changed_notify");
+	call_deferred(SNAME("ports_changed_notify"));
 }
 
 void VisualScriptCustomNode::_bind_methods() {