This changes how DokuWiki looks for reference toa media file which is
about to deleted. Instead of doing a full grep through all pages it now
uses the fulltext index first, then does an exact match on the found
pages.

This speeds up the search significantly on larger wikis. However the
fulltext search limits now apply: images with names shorter than 3 charcters
may not be found.

This needs extensive testing!

darcs-hash:20080223205254-7ad00-486de0a4125d51b4e7999827f710d1d9de8bc60d.gz