Editor On*GUI undo/redo example

EditorUndoRedoExample.cs

	protected virtual void OnSceneGUI()
	{
		HandlesTest testObject = (HandlesTest)target;

    // begin monitoring for GUI changes
		EditorGUI.BeginChangeCheck();
    
    // do whatever GUI stuff that lets user change something on the testObject
		var newPos = Handles.PositionHandle(testObject.transform.position, Quaternion.identity);
    
    // did anything in the GUI change? (ie was the transform handle moved by user)
		if (EditorGUI.EndChangeCheck())
		{
      // something changed, record that change for the object
			Undo.RecordObject(testObject, "Change SomePos");
      
      // apply the changed value to the object
			testObject.SomePos = newPos;
		}
	}