the old event tap has several problems, like no proper priority support
or having to set accessibility permissions for mpv or the terminal.
it is now replaced by the new MediaPlayer which has proper priority
support and isn't as greedy as previously. this only includes Media Key
support and not any of the other features included in the MediaPlayer
framework, like proper Now Playing data (only set dummy data for now).
this is only available on macOS 10.12.2 and higher.
also removes some unnecessary redefines.
Fixes#6389
der richter