places-view: implement a view for Other Locations

GtkFileChooser received a Other Locations view that lists
persistent devices, as well as networks and the root location
for the computer's hard drive.

Since Nautilus is a file management tool too, it should keep
consistency between Gtk+ file chooser, something that doesn't
happen since it doesn't display Other Locations.

To fix that, add NautilusPlacesView, a NautilusView implementation
that displays the GtkPlacesView widget. In order to implement that,
update window-slot to correctly display the places-view whenever
Other Locations is clicked.

https://bugzilla.gnome.org/show_bug.cgi?id=753871