BUG Make UI widget destroys more consistent to avoid exceptions.

Selectable would throw an exception in the GridField.js if destroy
called from onunmatch - at that stage jQuery UI would have had called
the destroy already. Add a guard, and change to onremove, which triggers
before the element is removed from DOM.
Add missing _super calls.
BUG DOM traversal fails after the element is removed from DOM.

Onunmatch triggers after the removal of the element from the DOM, which
makes DOM traversal fail. Use onremove instead, which triggers while the
element is still in DOM.