Support get_sort_key() in out-of-process volume monitors and gvfs-mount(1)

This is detailed in this bug

 https://bugzilla.gnome.org/show_bug.cgi?id=661711



Also extend the private D-Bus protocol used between the client- and
service-side for out-of-process volume monitors. In particular,
introduce a new a{sv} member that we can use in the future to for
expansion without breaking ABI.

Signed-off-by: David Zeuthen <davidz@redhat.com>