Our users tend to use muscle memory to pick from the view, this means it's common for things like: 1. type, DOWN, ENTER 2. type, mousemove, click These actions usually happen before the matches arrived, that means we should: a. Buffer those events and handle them once matches arrived b. Never replace a match if it's being mouseover-ed (or we could suprise the user by changing the expected destination. For these we need a bufferer and some smart events management.

Pushed by mak77@bonardo.net: https://hg.mozilla.org/integration/autoland/rev/adb5195b311e Implement a Quantum Bar event bufferer. r=adw