bug#32850: 27.0.50; window-swap-states doesn't swap window prev/next-buffers

[Juri Linkov]

> But if you are sure that it won't harm anyone to make it unconditional
> leave it well alone.  We can always add a customization later if
> someone needs it.  Personally, I don't care.

I'd like to make it customizable by using the existing customization in
the arg WRITABLE of window-state-get and window-persistent-parameters,
even though formally prev/next-buffers is not a window parameter (maybe
it should be, I don't know).

>> This patch handles killed buffers, so it works for desktop saving as well
>> (because the desktop restores the frameset only after it reads all buffers):
>
> Please install so we can see whether there are any problems (in
> particular with the desktop which I don't use).

Done, with a small change: even though set-marker is idempotent in regard
to its POSITION arg (i.e. if POSITION is a marker, it creates an identical
marker), I added a check to not create a new one.  OTOH, get-buffer is
idempotent too, but it seems window-state-put never receives a structure
with buffer objects, and I'm not sure why window-state-get should always
use buffer-name regardless of the value WRITABLE, i.e. why should it return
buffer names as strings instead of buffer objects even when WRITABLE is nil?