media reference check

This patch implements the first step of a media file reference
checker. Every time the user wanted to delete a media file
it would be ckecked for still existing references to this media
file. File deletion is denied if this media file is still in use.

darcs-hash:20050605185038-7ef76-475e5990609587e1b8cee0e155fa6002f1c5b27c.gz