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!