?: to be discussed, /: in progress, x: done, 1: 2.1

[x] InterfacePlugins stuff
  [x] UIEventLoop class, used by InterfacePlugins to encapsulate common tasks, like:
      - sending visualization messages to vis plugins
      - GTK
      - DBus
[1] Playlist rewrite
[x] Output plugin API revisit
      - init failure/success
