Allow retrieval of FuncRef function string

(cherry picked from commit 75154e64a9)

core/func_ref.h

@@ -47,6 +47,7 @@ public:
 	Variant call_funcv(const Array &p_args);
 	void set_instance(Object *p_obj);
 	void set_function(const StringName &p_func);
+	StringName get_function();
 	bool is_valid() const;
 	FuncRef();
 };