Search widget key commands

Objective:

* Enable navigating search results using keyboard up and down keys
* Bubble up query input enter events

Changes:

ve.ui.SearchWidget.js
* Route query input events to emit
* Add handler for keydown events
* Support navigating through the list of results using vertical arrow keys

Change-Id: Ia2e4b27075a8ab2e29a69294e9f7847e8f5a3f83
Merge "Search widget key commands"