Use flat tray menu

Shows only the leaf devices as submenus. These submenus inherit all
their ancestor device methods.
Automatically lock LUKS slave when unmounting from tray menu

This results in the following race condition: If the notification
service is run in the same daemon, no unmount notification is shown
because the device is locked and therefore unavailable when the change
event is handled.

It might be better to use the automounter daemon to automatically lock
unused LUKS devices. This centralizes the responsibility and could solve
the notification issue.