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
in your patch you recursively call `destroyWidgets` here but i believe that doesn't work because the patch algorithm is going to skip those patches due to not having a related `domNode`.  instead, you have to add a `VPatch.THUNK` patch with sub-patches for them to be applied.  i see you've tried to adjust the indexing to make this work but it's still not right.