The most used feature of any image editor, undo. Each tool now
notifies the ImageEditor that they completed an action, where
it'll take a snapshot if its current state.
For now, a snapshot is just a copy of the whole image and its
layers. There's a hard limit on the amount of actions it stores.
We use this to automatically update the thumbnail in LayerListWidget
when you draw into a layer. We also use it to repaint the ImageEditor
when the image changes somehow. :^)
