bug#10348: 24.0.92; Save and load window states

[martin rudalics]

> And of course the parameters preserved intra-session is a superset of
> the parameters preserved inter-sessions.

I'm not so sure.  Maybe a parameter is useful intra-session but not
readable from disk.

> Using window-state-ignored-parameters to specify the difference between
> the two sets seems OK.

You mean that any value present in `window-state-ignored-parameters'
would have to be a member of the `window-persistent-parameters' I used
in my patch?  I'd rather not care about
`window-state-ignored-parameters' in `current-window-configuration' or
`set-window-configuration'.

(BTW, I use `window-persistent-parameters' instead of
`window-state-saved-parameters' because they apply to both,
`current-window-configuration` and `window-state-get'.)

> An alternative would be to ignore
> window-state-saved-parameters (and window-state-ignored-parameters)

So far, I didn't care about `window-state-ignored-parameters' when
restoring a state in `window-state-put'.

> upon restore.  Instead, we'd save the parameters in a list where each
> element is either (PARM . VAL) or just PARM where the second form
> indicates that PARM was not set and should hence be unset upon restore.

Fine with me.  Obviously, this makes saved states a bit larger.

martin