lyrics: converted in-process plugins to external programs

lyrics: converted in-process plugins to external programs

In-process plugins are very problematic.  It is much easier and
flexible to move the lyrics plugins to external programs, with a
trivial protocol.  This is work in progress, among the things missing:

 - protocol specification, including exit codes
 - plugin installation
 - plugin search directory
 - run-time configuration (currently hard coded)
 - automatic polling (using glib's main loop?)
 - better and more robust error handling