Remove from clickable_elements on remove

The SWT app tracks things we can click on for changing the mouse cursor via
the clickable_elements ivar.

When we removed elements, they weren't getting taken out of that list. This
included calls to `clear` on slots.

Note that the Shoes::Swt::Common::Remove module currently didn't have any
specs, and this felt like a spec for it would just reiterate exactly the
production code so I left it off. If anyone disagrees strongly, I can add that
in.

Fixes #831