Show a notification when we receive a show-unmount-progress signal on
the mount operation we use for unmounting.
The notification will either turn fade out automatically with a
completion message when the unmount successfully completes, or will
disappear in case the operation is aborted underway (for example because
the device has been unplugged in the meantime).
https://bugzilla.gnome.org/show_bug.cgi?id=676125