Bug 840160 - Selecting to enter edit mode on an app that is downloading will disable the ability to launch any app until the phone is restarted or upon entering edit mode on a non-downloading app r=crdlc

* optimize png sizes
* add basic tests for grid
* always enable panning when the user presses the home button
* don t spin the icon while drag&dropping
* always create a renderedIcon, even in the downloading state
* add the img as late as possible to the DOM
* change GridManager from const to var to be able to mock and test it
(cherry picked from commit 871aba5cbdb50dc48b352dbb0be5474f44e4a09c)

Conflicts:
	apps/homescreen/js/grid.js
	apps/homescreen/js/page.js