cocoa-cb: re-enable `wid` (NSView) embedding on macOS for libmpv use

[softworkz]

In version 0.37, support for the old cocoa backend had been dropped, which unfortunately also removed the ability of embedding mpv in native applications by specifying a wid - which is still working for all other platforms.

This PR aims to remedy that situation by leveraging the existing cocoa-cb implementation which already has just about everything that it takes and it has been merely about gluing loose ends together and omitting those parts that are specific to standalone application use, like NSApplication and NSWindow.