Bug 1043599 - Show app titlebar after swipe in fullscreen apps

https://bugzilla.mozilla.org/show_bug.cgi?id=1043599
you're not doing the `getTopMostWindow()` here, so launching a inline activity to Messages from the share button of the gallery will get you in a weird state where the title bar doesn't go away.

We might want to store the manipulated element in a `_currentBar` var.