- split DObject::Destroy into the main method, a native OnDestroy and a scripted OnDestroy method and made the main method non-virtual

This was done to ensure it can be properly overridden in scripts without causing problems when called during engine shutdown for the type and symbol objects the VM needs to work and to have the scripted version always run first.
Since the scripted OnDestroy method never calls the native version - the native one is run after the scripted one - this can be simply skipped over during shutdown.

src/dobject.h

@@ -473,7 +473,8 @@ public:
 	// that don't call their base class.
 	void CheckIfSerialized () const;
 
-	virtual void Destroy();
+	virtual void OnDestroy() {}
+	void Destroy();
 
 	// If you need to replace one object with another and want to
 	// change any pointers from the old object to the new object,