Basic undo demonstration

Hey,

Thought you might find this interesting to see. I've only done it for the GuideMarker drag and drop creation but is seems to work (without looking too closely) and shows the basic premise of the UndoStack setup.

The system can get confused if some stuff is redoable and some stuff isn't so it might be worth implementing everything in one big go but on the other hand it might not be the end of the world if only bits work for the moment. Just saying I don't really expect you to merge this in now.

It is also worth bearing in mind that there is a QUndoStack and QUndoCommand which might be worth investigating and using if there are necessary Qt hooks in there. I'm not sure myself. 

gg wp
Mike