bug#44973: Add a macOS global hotkey function

[j]

MacOS makes it very difficult to assign global hotkeys to focus Emacs. For example, if I'm in another program, and I want to launch/focus Emacs with "Alt-SPC", it can't just be set in the system preferences; I'd need to use a 3rd-party program like skhd.

Would it be possible to add a function or hook to Emacs which registers one or more global hotkeys (using addGlobalMonitorForEventsMatchingMask), and then when the hotkey is pressed, run an elisp function?

For example, by creating a "ns-register-global hotkey" function, where the user could choose a keybinding (maybe "ALT-SPC"), and letting the user provide a hook function (maybe "org-capture-stuff")

I intend this feature to be used for something like an Alfred replacement, as described here: https://www.mattduck.com/emacs-fuzzy-launcher.html .

Even if it's rare for Emacs to implement an OS-specific feature, there is some precedent on Windows. If you look at w32-register-hot-keys, it's similar to, but different from my request. 

The implementation in nsterm.m should be simple, but it requires Obj-C and Emacs-specific development knowledge.

Thanks!