osx: improve Media Keys support

This commit addresses some issues with the users had with the previous
implementation in commit c39efb9. Here's the changes:

  * Use Quartz Event Taps to remove Media Key events mpv handles from
    the global OS X queue. This prevents conflicts with iTunes. I did this on
    the main thread since it is mostly idling. It's the playloop thread that
    actually does all the work so there is no danger of blocking the event tap
    callback.
  * Introduce `--no-media-keys` switch so that users can disable all of mpv's
    media key handling at runtime (some prefer iTunes for example).
  * Use mpv's bindings so that users can customize what the media keys do via
    input.conf. Current bindings are:

      MK_PLAY cycle pause
      MK_PREV playlist_prev
      MK_NEXT playlist_next

An additional benefit of this implementation is that it is completly handled
by the `macosx_events` file instead of `macosx_application` making the
project organization more straightforward.