universal callback for search()

Ignore-this: 4d786345ea9bfb19fb6f8af9348f5248

This patch adds a callback for use with the search() function,
that is flexible enough to replace many of the other specialized
callbacks and opens up more possibilties to plugin authors without
the need to write new callbacks.

Existing callbacks need to be reexamined and rewritten to wrap
around this callback instead.

note: development was part of the ICKE 2.0 project see
      http://www.icke-projekt.de for info

darcs-hash:20090812101653-6e07b-d578baff97a0e8720e468e238aec06109048c8b8.gz