Add G_VOLUME_IDENTIFIER_KIND_CLASS identifier

If an application (such as Nautilus) wants to show a sidebar with
devices group into different groups such as "Devices" and "Network",
it's currently up to the application itself to do the classification
(for example by looking at the URI scheme for the activation root,
e.g. smb://).

This patch adds a new identifier G_VOLUME_IDENTIFIER_KIND_CLASS that
can be set by volume monitors and used by applications.

See https://bugzilla.gnome.org/show_bug.cgi?id=668295



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