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.
Spec for Shoes::Swt::Common::Remove
Merge pull request #862 from shoes/clickable_elements_on_remove

Remove from clickable_elements on remove

Changelog: bugfix