Nowadays, we rely on G_FILE_MONITOR_EVENT_UNMOUNTED to be emitted when a
mount disappears, since we already monitor the directory, in order to
switch the view to a different location.
Since non-native mounts very likely won't have file monitoring though,
we're not going to receive such events in that case, and fail to
redirect the view as a consequence.
This patch fixes it by listening to the mount-removed signal on
GVolumeMonitor when a monitor is requested for a non-native mount, and
emulating a file removal in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=684226