Clean-up the selection view when removing it

When a cursor is removed from the document (e.g., exiting edit mode),
the most recent selection is left visible.

* Don't re-append the overlayBottom on destroy
Upgrade from magical empty-string to const for shadow cursor memberId

In various parts of the code, the magical empty-string cursor id is used
to determine whether the object is a shadow cursor or not. Converting
this into a constant instead and removing the constructor arg prevents
some mistakes being made with this.