There are some issues with the existing session menu. First, it looks
kinda bad. It seems like it's hanging around there, but it doesn't really know
what to do with itself.
Second, when it expands down it requires that the buttons below move
down with it. This kind of movement is awkward and looks a bit weird.
Third, its current position makes the "dialog" tall and unwieldy when
you add things like messages for fingerprint readers or authentication errors.
This commit moves the session list to a menu behind a button to address
the above problems.
Based on a patch by Jasper St. Pierre.
https://bugzilla.gnome.org/show_bug.cgi?id=702818
8d7649eaec
