Warn about singleton being a Reference

(cherry picked from commit e1150bd912bd6076b8a94544c0ec32f75344976e)

core/engine.cpp

@@ -236,3 +236,13 @@ Engine::Engine() {
 	_frame_step = 0;
 	editor_hint = false;
 }
+
+Engine::Singleton::Singleton(const StringName &p_name, Object *p_ptr) :
+		name(p_name),
+		ptr(p_ptr) {
+#ifdef DEBUG_ENABLED
+	if (Object::cast_to<Reference>(p_ptr)) {
+		ERR_PRINT("A class intended to be used as a singleton must *not* inherit from Reference.");
+	}
+#endif
+}

core/engine.h

@@ -42,10 +42,7 @@ public:
 	struct Singleton {
 		StringName name;
 		Object *ptr;
-		Singleton(const StringName &p_name = StringName(), Object *p_ptr = NULL) :
-				name(p_name),
-				ptr(p_ptr) {
-		}
+		Singleton(const StringName &p_name = StringName(), Object *p_ptr = NULL);
 	};
 
 private: