cocoa: remove usage of Objective-C categories

Objective-C categories need special linker flags from the user when statically
linking (-ObjC LDFLAG), so make everyone's life simpler and remove them.
libmpv/cocoa: allow clients to use mpv event system

This allows mpv's view to take key and send events to mpv's core.
To set key status correctly, clients must call -[NSWindow selectNextKeyView:]
during reconfig on the main thread. All is 'documented' in the cocoabasic
example.

If someone knows a better way to handle giving key to the embedded view,
let me know!
vf_vapoursynth: abstract scripting backend

In theory, vsscript should be doing it, but it's not there yet, neither
did there seem to be any interest in making it flexible enough to handle
more than 1 scripting language.
vf_vapoursynth: add standalone Lua scripting
gl_x11: distinguish missing GLX and too old GLX versions

This probably led to confusing output.

CC: @mpv-player/stable
DOCS/edl-mpv: minor clarification