Change the workflow for image pixelation

[blackstream-x]

• Add an undo stack of 10 (or 20?) saved images
• enable or disable the undo, apply and save button by traced tkinter variables
• Save an applied state only. Set the "touched" flag only after applying.
• Ask positively on exit: "Save changes …?" instead of "Discard changes …?"
  • If "Save changes …?" was answered with "yes", and the save dialog is cancelled, ask for "Exit without saving?"
• Same behaviour, but other wording when loading a new image and the current is "touched"