Finish the playlist API transition (hopefully)

 - Remove a bunch of transition now-useless API.
 - Remove all playlist_Lock* functions. Matching functions now accept a
   b_locked parameter that should be set to VLC_TRUE if the playlist is
   locked when calling the function. The advantages are that it reduces
   the API bloat and that the b_locked flag can be propagated in the
   call stack
 - Remove useless playlist testsuite
 - Remove broken daap plugin (unmaintained wrt VLC API changes, relies on 
   an unmaintained library, probably unable to read content from new itunes, 
   ...). Implementations exist in rhythmbox, xmms2 and 
   daap-sharp, we should see if a proper lib exists or if we could 
   make one
 - Reduce verbosity a bit
 - Documentation