* src/playlist/playlist.c:

* src/playlist/playlist.c:
  - added an intf-change variable. This is set whenever you add or delete
    an item in the playlist. You need to unset it yourself.
* modules/gui/macosx/intf.?:
  - listen for external playlist changes. update view when playlist changes.
  - activate/deactivate previous and next buttons and menuitems,
    whenever the playlist gets changed.