handle destroying widgets in thunks

* fix airgap for hasWidgets so that it's set on the parent of thunks if
  needed
* handle thunks when identifying widgets to be destroyed
we probably should but the reason we don't is due to an inconsistency.  vdom handles thunks during an insert (https://github.com/Matt-Esch/vdom/blob/master/create-element.js#L16) but thunks are not handled during a remove patch.  i think it would probably be more correct NOT to handle thunks in create-element and provide all the right patches in the first place.  i.e. i think `VPatch.THUNK` should be the only path that handles thunks but currently `VPatch.INSERT` (and maybe others) handles thunks.