Simplify slug behaviour

By removing the ability to cursor out of an empty slug and have it
delete instantly, and by adding a click handler to the slug instead of
trying to observe the cursor ending up inside it, we greatly simplify
the code around slug handling.

Bug: T89192
Bug: T74494
Bug: T85014
Bug: T67742
Change-Id: I3683d0f706f17258fb76c980aba6e53777d59831