bug 975850

* Stack Manager should start up with _current set to -1 since we launch the homescreen by default.
* Cards View should show most recently used application (last on stack) when the homescreen is being shown.
* Added tests for showing cards view from homescreen in default running state (eg. no recent apps)
* Added tests for stack manager to ensure that position is always -1 initially.
* Seperated the tests for cards view active, one set for in the rocketbar, one set for outside of the rocketbar.
** The original test covered the default running state case, it was updated for the task manager being shown in
   rocketbar but we did _not_ backout that change when disabling the rocketbar in 1.4.
* call blur on AppWindow instead of on frame.

r=alive
Merge pull request #16580 from nullaus/bug975850-cards-view-stack-manager

bug 975850