bug 981764

* Add 'cardviewbeforeshow' and 'cardviewbeforeclose' events to cards view.
* Added handling of 'cardviewbeforeshow' and 'cardviewbeforeclose' to HomescreenLauncher
  so that it will now fadeOut the homescreen before the card view is shown and fadeIn the
  homescreen before the card view is closed.
* Added tests for new homescreen launcher behavior.

r=alive
Merge pull request #17607 from nullaus/bug981764

bug 981764