Text input widget enter event

Objective:

* Add an event to single line text input widgets to detect when the enter
  key has been pressed

Changes:

ve.ui.TextInputWidget.js
* Retain value of multiline config option
* Add key press handler to detect pressing of enter key
* Emit 'enter' event when enter key is pressed if input is single line

Bonus:

* Add missing documentation for multiline config option

Change-Id: Id9c64343f4a2ea8f0f45213fd2b59ca87c805b24
Merge "Text input widget enter event"