drivemount: regression: audio CDs no longer displayed

since dapper, drivemount no longer displays in the list of drives CD
drives containing audio CDs.
I think it's because it has been decided to hide drives that the user cannot
mount.

In the case of audio CDs, it's a regression IMO, because:

1. you can no longer click the drive and select "Play CD", option which was
introduced in 2.12, so beginner users will have as only solution to
eject/reintroduce the CD to get it to play if they don't know which program is
used to play audio CDs.

2. you can no longer eject the CD using the drivemount applet. you must use the
button on the CD drive. imagine explaining to a beginner that he must use the
drivemount applet to eject CDs... unless if it's an audioCD. then he must use
the button on the drive. it's confusing.

I think the new behaviour should be changed: don't display drives that can't be
mounted, unless they are audioCDs. Always display audioCDs (like in breezy).

I first reported this bug in GNOME bugzilla but it seems it's ubuntu specific so I report it here.