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.
all the get_property_* usages were removed because in some circumstances
they can lead to deadlocks. they were replaced by accessing the vo and
mp_vo_opts structs directly, like on other vos.
additionally the mpv helper was split into a mpv and libmpv helper, to
differentiate between private and public APIs and for future changes
like a macOS vulkan context for vo=gpu.
der richter