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.
