Better initial sessions

When I finetuned the initial sessions, I was glad to have implemented keyboard support on PCs. To move points horizontally only, you simply hold down the shift key. For vertical movements you can use the ctrl key. When doing the refinement I often needed to do an undo. I then always pressed ctrl-z like a reflex and was then annoyed, that I had to press the undo button. So I implemented this key to, as well as the ctrl-shift-z for redo.