Bug 935260 - Rework cards view as TaskManager and Card/TaskCard classes

Addressing review comments:

* Dont return self from start method
* Guard against no element in enterTaskManager, clear out .app and .manager references in Card.prototype.destroy()
* Removed dead code and tidy up z-indexing selectors
* Use bind instead of self=this
* Implement and use transform method on AppWindow
* Implement unapplyStyle, move settings change observer to registerEvents
* Store GestureDetector on the constructor to ensure we only ever instantiate one
* Fix up unit tests
* Use rocketbar.enabled setting to toggle isRocketbar property; unit test same
* More unit tests for isRocketbar behavior, testing setActive, hide
* allowManualClosing property to allowSwipeToClose, get rid of unused inTimeCapture, lastInTimeCapture
* Move opacity, transition, scale constants to card prototype. Use card.applyStyle to avoid touching the element.style directly. Fix pointer-events so current card should always be clickable. Fix adjacent card positioning so they occupy the 25% at the edge
* Adding unit tests for AppWindow apply/unapplyStyle, enter/leaveTaskManager and transform methods
* Add unit tests for card.destry and taskManager.closeApp
* Resolve merge conflicts and add mock TaskManager back in to bootstrap test
* Twiddle dials on task-manager unit tests to better reset state between tests

* Adjust swipe_to_previous_app in cards_view ui-tests to start swipe from center
* Fix applyStyle to delete a property when the given value is undefined, which fixes the ui-test