Bug 958251 - Task Manager

User accesses Task manager by swiping down from the top of the display.

Features:

* A task manager is displayed when you pull down from the top of the screen. (Rocketbar must be enabled).
* The default rocketbar view remains when you tap on the statusbar.
* The task manager will show a list of cards, each card corresponding to an active application or browser.
* Rocketbar title is updated with each card title as you swipe through cards.
* When focusing on the title, the title should clear if it's an application.
* When focusing on the title, if the title is a URL, it will remain populated.
* The cancel button is hidden while in task manager mode.
* When pressing cancel after focusing, the view will revert to showing the task manager.
* When pressing cancel after focusing, the rocketbar will close if there are no open apps.

https://bugzilla.mozilla.org/show_bug.cgi?id=958251