Trigger redraw on hover/leave for slot

To get a proper redraw after the hover/leave on a slot, we needed to pull
the proc execution into the mouse_hovered/mouse_left methods so we could
hook those with the RedrawingAspect.

This moves the specs for this to the DSL level since the `Shoes::Slot` now
takes care of executing the procs that it's holding as well.

Changing an image path wasn't triggering a redraw, so added that to the
update list as well.

Fixes #835.