move all backend independent code parts in their own folder and files,
to simplify adding new backends. the goal is to only extend one class
and add the backend dependent parts there. usually only the (un)init,
config and related parts need to be implemented per backend. furthermore
all needed windowing and related events are propagated and can be
overwritten. the other backend dependent part is usually the surface for
rendering, for example the opengl oder metal layer.
in the best case a new backend can be added with only a few hundred
lines.
this basically moves the remote command center to our mac events instead
of keeping it our Application, which is only available when started from
mpv itself. also make it independent of the NSApplication.
this also prevents a runtime crash
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