GtkPlacesView exposes local access points and network
shares transparently by using the 'network:///' URI,
which is handled by GIO.

Currently, however, it doesn't monitor the network
for new available points, such as computers that just
join the network. It may happen too that the backend
won't find all the networks before the network enumeration
finishes.

Fix that by keeping a file monitor inspecting the network
uri, and update the places list when that happens.

https://bugzilla.gnome.org/show_bug.cgi?id=781162