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.