#3 Audio CD ripping

[Cliff L. Biffle]

Songs should be able to be dragged from an audio CD into a
DirSource. This will cause them to rip directly to disk using the
user's rip preferences.

We -may- be able to use audiocd: for this, but it's awfully limited
(only one drive, etc.).

If we write our own ripper code (or even if we don't) we should
definitely keep it modular, since it's likely to change later.