bug#44973: Add a macOS global hotkey function

[j@mremus.net]

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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!

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