#5 Generic removable media support

[Cliff L. Biffle]

We should cope gracefully with filesystems being remounted or
rearranged, at least in the case of removable devices (and
possibly in all cases). Ideally, we would see situations like:

- The user inserts their USB pen drive filled with music. They've
already configured hotplug to automagically mount their drive,
and they've shown Kiwi the drive before. When the drive is
mounted, Kiwi would respond by showing a new Library option in
the playlist box, containing all music on the drive. When the
drive is removed, the option disappears. While it's available,
drag and drop and such work as expected.

- The user inserts a CD of MP3s. We're keeping an eye on the
drive for audio CDs, but this isn't one, so we ignore it. When it's
mounted (we probably shouldn't try to mount it, as we're not
root) we provide a Library option like for the removable pen
drive. We'll want to look into CD packet writing support on
Linux/BSD to see if dragging files to an existing CD should do
anything.

- The user mounts their remote music server over NFS. Same
deal.

This will be a variant of the venerable DirSource class, but will
need to monitor the mountpoint and distinguish between a
(possibly infinite) variety of filesystems that can be mounted
there.