Removed a lot of no longer needed cruft.

 * Navigation (Clockwerk playlist supported the notion of navigating
   like in a DVD menu.)
 * Networking (synchronizing the clip library with a server, which is
   not open sourced)
 * Schedule management (Playback units could be programmed on a weekly
   schedule.)
 * Not useful clip types
Removed Executer clip type and renderer

 * It was used to execute a command line app at a specific time in the
   playlist.