Tools can now have an in-image context menu which you get when right-
clicking inside the image editing area. Our first use of this is to
provide the ability to move layers to the back/front. :^)
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.
Tool mouse event handlers now receive both a layer-relative mouse event
and the original event. This is needed for the move tool since it moves
the layer and thereby changes the origin of future events every time it
moves.
These two have the same semantics as GUI::Widget. The rect() is always
at location { 0, 0 }, while the relative_rect()'s location is relative
to the "parent", which in this case is the Layer's Image.
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.
