Show applications using the message tray, and
allow configuring in what way the shell presents them.
The set of applications shown include all applications that ever
showed a notification in gnome-shell and all applications that have
a boolean X-GNOME-UsesNotifications key set to true in their desktop file.
https://bugzilla.gnome.org/show_bug.cgi?id=685928
Now that we don't allow or load external panels, using libgnome-menu is just
overengineering. We can get the same results with less code by keeping a static
list of function pointers.
This reduces the number of places one needs to patch to add a new panel.
Also, this way we avoid registering all types at startup, and if we want
we can switch to load panel desktop files in a separate thread.
https://bugzilla.gnome.org/show_bug.cgi?id=690165
