We now use the "active" property of views to track whether navigation is possible. This has the advantage of being already handled correctly when views are moved to a different panel, and is in line with the purpose of the "active" state. The note about using the "popupshown" event for navigation has been updated accordingly.
Keyboard navigation is also linked to the "active" property now, so there is no need to track the state of the "_transitioning" property anymore.
Since the goBack and showSubView methods can only be called when the view is active, we don't need to check for attempts to start a transition while the panel is closed anymore.
MozReview-Commit-ID: 3KT3A5EwGFy
