Change the way art is stored and retrieved

This patch was inspired by the album art problem reported [in the forums](http://rainwave.cc/forums/viewtopic.php?t=4172). *Dead or Alive* has album_id 5, which matches the All channel sid. *Xenogears* has album_id 120, which matches one of the album art size points.

With this patch, the scanner will store all art without any reference to sid. Each album will have the same art on all channels.

All stored art will always specify art size. Each album will have three art files, \<album_id>_120.jpg, \<album_id>_240.jpg, and \<album_id>_320.jpg. The front-end is modified to look for different art sizes according to these rules.

If you accept this patch, I recommend you delete all the existing stored art and rescan for all art files.