Prevent empty widget addition from being flagged as unsanitary
Prevent widget element lookup race condition when moving widget to another sidebar

* scope widget element lookups by associated sidebar
* instead of appending to the parent, we should append relative to the first widget found
Improve logic for cleaning up when widget is removed from sidebar

* Remove required preview refresh when widget is moved to another sidebar
* Ensure that widget moved to other sidebar without drag/drop isn’t trashed