This feels very intuitive and nice, although maybe we need some way to
override this behavior for the scenario where you're intending to move
something currently behind something else.
You can now see what you're drawing before committing to it. This works
by passing the second_paint_event from the ImageEditor to the tool.
We also pass the active layer which makes it easier for the tool to
keep his logic in layer-relative coordinates even while drawing preview
states directly into the ImageEditor backing bitmap.
The main editing widget is now the new ImageEditor widget, which works
on an Image object, which internally has a stack of Layer objects.
Layers are composited back-to-front when painting the Image inside an
ImageEditor.
