Namespace Browser should have notification when we leave a view

[aghassemi]

Currently in the NB there is no way to know when the user leaves a view. So, for example, if NB is in the tree view and it is loading additional children, if the user switches to a different view the tree view keeps loading children. There should be some kind of internal notification when the user leaves a view, so the ex-view can shut things down.

If Mercury doesn't have a way to automatically supply this, we should just detect it manually when the user presses one of the three view buttons (or the bookmarks or recommendation buttons).