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

[Drew Adams <drew.adams@oracle.com>]

> > And maybe print the point values too, unless they're the same
> > buffer?
> >
> > Different, A: 2349, B: 1358
> > Same: A: 2349, B: 1358
> > No match
> 
> Or the size of the region of difference or sameness.

Maybe.  Or both:

Diff: 623 chars, A: 2349, B: 1358
Same: 623 chars, A: 2349, B: 1358
No match

> > Dunno what the names might be.  They could be the buffer names
> > (possibly truncated, if long), if different.
> 
> There is no displayable information associated with windows
> other than buffer names.

Yes, but names can be assigned.  In this case, only two names
are needed.  Just as for Ediff, we could use A & B, e.g., A
for the selected window.

> >> 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'.
> 
> There are still open questions about this new variable, e.g.:
> what should be recorded in `last-selected-window' in case of
> `C-x 0' (`delete-window') when the last window is deleted?

Pick one.  The (newly) selected window, for example.  Or reset
it to nil or whatever (i.e., none).  Selected window makes
sense, I think.

Unless we are going to record a whole history, as for buffers,
we cannot set it to the previous "last".  Imagine five `C-x 0'
in a row.