Emit select null on selected item removal

Issues:

* Selected item is removed, but no select even was emitted

Changes:

ve.ui.SelectWidget.js
* On remove, emit select null when removing a selected item
* On clear, emit select null always

Change-Id: I1a94b0f8a841611b20e09f6c02f37fb9589e85c3
Merge "Emit select null on selected item removal"