Bug 1089615 - Fade to white background when showing browser-windows switcher. r=etienne

* Apply filtered class on task manager earlier
* Pass filter in cardviewbeforeshow event to enable fade out to white in homescreen.
Merge pull request #25554 from sfoster/tm-early-filter-bg-bug-1089615

Bug 1089615 - Fade to white background when showing browser-windows switcher. r=etienne, a=fabrice