bug#44973: Add a macOS global hotkey function

[Eli Zaretskii]

> Date: Mon, 21 Dec 2020 08:12:04 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: 44973@debbugs.gnu.org, Lars Ingebrigtsen <larsi@gnus.org>
> 
> One potential problem with your approach is that we don't accept code
> using Objective C blocks as it's incompatible with GCC. Unless that
> policy has changed?

It hasn't changed, but does this code really need to use that feature?
The last time we had a much more grave problem: the system header used
for some code itself used ObjC blocks, and thus couldn't be compiled
by GCC.  It looks like this case is not that problematic?