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

[Juri Linkov <juri@linkov.net>]

> How to choose?  Click the mouse in a window,

This is what `ediff-get-window-by-clicking' does.
But what to do if the user doesn't use the mouse?

> or choose a window by name using the keyboard -
> those are two possibilities I suggested.

You mean to choose a window by it's buffer name?
I.e. like `ediff-buffers'?  But the problem is that the
uniqueness of buffer names is not guaranteed in this case.

What do you think about such workflow: select a target window
using normal keybindings or mouse clicks.  Then mark the
selected window as the target window using a new prefix
of `compare-windows'.  Then switch back to the original window
using standard keybindings, and run `compare-windows'.
It could use the remembered target window.