bug#42028: [Feature Request] 27.0.91; Provide the ability dynamic modules to post events in emacs event loop

[Eli Zaretskii <eliz@gnu.org>]

> From: Ivan Yonchovski <yyoncho@gmail.com>
> Date: Wed, 24 Jun 2020 10:14:44 +0300
> 
> This is needed for the cases when the module is listening for extenal
> events and wants to call back emacs to process them. ATM this is kind of
> possible by using signals on linux and using WM_INPUTLANGCHANGE as
> described in https://nullprogram.com/blog/2017/02/14/ but this looks
> more a hack for a missing feature.

It is almost trivial to let modules insert events into the event
queue.  The problem is that I expect a module that inserts such events
to want to be called to process those events as well.  Is that
expectation correct?  If it is, then we need to think about extending
the mechanism that calls event handlers, not just about exposing the
likes of kbd_buffer_store_event to modules.  We should also consider
how a module could define its own events.

IOW, this calls for a slightly more detailed specification, before
someone could sit down and code the stuff.  I suggest to take a look
at how existing events are handled, and propose such a detailed
specification.

Thanks.