Optimize Object::get_class_name

* Run the static function once per class instead of one per instance.
* Saves some memory in Object derived classes.

core/string/string_name.cpp

@@ -201,6 +201,14 @@ StringName::StringName(const StringName &p_name) {
 	}
 }
 
+void StringName::assign_static_unique_class_name(StringName *ptr, const char *p_name) {
+	mutex.lock();
+	if (*ptr == StringName()) {
+		*ptr = StringName(p_name, true);
+	}
+	mutex.unlock();
+}
+
 StringName::StringName(const char *p_name, bool p_static) {
 	_data = nullptr;