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

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

Provide support for `compare-windows' to compare two windows in
different frames.  Ediff lets you compare buffers in different frames.
It would be great if `compare-windows' did also.

Yes, this would require users to choose the windows, instead of just
using the selected window and the next window.  This could be optional.

Or it could be solicited by, e.g., using a particular prefix arg value.
E.g., with a non-negative prefix arg, do what the presence of any prefix
arg does now.  With a non-positive prefix arg, prompt the user to choose
the 2 windows.  (So a zero prefix arg would do both.)

Choosing a window could take any reasonable form: click mouse-1 in it,
choose by its buffer name (with uniquification when multiple windows
show the same buffer),...

Successive calls (with no prefix arg) should, as now, keep using the
same two windows.

(None of the current behavior should be lost, in particular, behavior
per `compare-windows-sync'.)

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'