StackLayout: Clean up inconsistent event firing and documentation

* Document the unset side-effect in setItem.

* Consistently fire the "set" event when unsetting the current item
  (like #setItem did already).

* Add missing @fires to methods.

* Remove stray "[description]" placeholder added in If1645158a.

Change-Id: I7265c06335e1f53f9ad2c11c386c569f9b70cdfc