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.
Redraw on image loading

Changing an image path wasn't triggering a redraw, so added that to the
update list as well.
Merge pull request #858 from shoes/hover_redraw

Trigger redraw on hover/leave for slot and image update

* Changelog: improvement