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
* don't hide the icon if it's already rendered-ready
* add the img as late as possible to the DOM
* change GridManager from const to var to be able to mock and test it