filechooserwidget: use places view to manage fixed devices

filechooserwidget: use places view to manage fixed devices

Previous patch modified places sidebar widget to stop handling
fixed devices by adding an "Other Locations..." item. Up to now,
however, these changes are isolated from each other since the
bundled file manager widgets ignore the sidebar requests for
external management of fixed devices and networks.

To fix that, make the file chooser widget be aware of the
GtkPlacesSidebar::show-other-locations signal and, when requested,
show places view to manage the fixed devices and networks.

https://bugzilla.gnome.org/show_bug.cgi?id=752034