bug#52235: 29.0.50; Suggestion: refactor time.el into a more general 'clock' framework

[Arthur Miller]

Last few days I have being playing with a clock just to test the transparency,
and what I have noticed is that I don't see some framework to work with clock
time. For tests I have used timer + 60s to get per minut update, but that is of
course skew time.

Emacs has a better "clock" framework in form of 'display-time-mode', but it is
explicitly coupled to mode-line updates. So any application that would like to
per-use it, will also trigger mode-line re-display, regardless if user have
'display-time-mode' on or not.

My idea to per-use it is to simply run 'update-hooks' on clock beat, so any
application can register a hook that will be called on a "clock beat". While it
is maybe not so super-synchronized with system clock, it still gives impression
of clocks in Emacs be synchronized.

Changes look bigger than they are, but I have made some let-bound variables
global, so I can use them as precalculated variables instead of calculating them
in the application code. I am not sure if it is correct. but it's just a test.

Allaround naming is bad, but I tried to be completely backward compatible, so I
had to preserve the old hook/var/mode names.

Example usage is in attached emvac-vision-clock.el, with attached screenshot of
it running.

I haven't thought of how/when to kill timer and internal minor mode properly; I
guess when hooks variable could be tested for nil, but I haven't done it yet.

time.el.patch
Description: Text Data

emacs-vision-clock.el
Description: Text document

clock-on-refactored-time.el.png
Description: PNG image

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo 
version 1.17.4)
 of 2021-11-28 built on pascal
Repository revision: f5498a608fe85b66a4068a5ccf88c9b3d3b88f98
Repository branch: private
Windowing system distributor 'The X.Org Foundation', version 11.0.12101001
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