bug#19170: 25.0.50; enhancement request: `compare-windows' use across frames

[Juri Linkov]

> Some things that I think are missing, or that might be helpful:
>
> 1. Maybe echo some indication of which two windows are compared (?).

Currently `compare-windows' displays in the echo area "Mark set"
which is mostly useless.  We could overwrite it with more specific
messages, e.g.

  Next difference between window1 and window2
  Next match between window1 and window2
  No more matching points between window1 and window2

> I'm thinking that having that last-selected window recorded
> always might be a good thing, useful for more than just
> `compare-windows'.  (And perhaps (?) the NORECORD arg of
> `select-window' should control updating of `last-window-selected'
> also.  Dunno.)
>
> What do you think?  Does this make sense?

We already have a lot of `last-...' variables, so a new variable
for the last selected window makes sense.  Then it would be
convenient to compare `(selected-window)' with `last-selected-window'.