X-Debbugs-Cc: Juri Linkov <juri@linkov.net>

This adds a capability to customize the destination window selection 
logic for navigation (xref-find-definitions, xref-go-back, 
xref-go-forward) by allowing a user-supplied display window function.

Inspired by the Merlin package and its user option 
merlin-locate-in-new-window 
(https://github.com/ocaml/merlin/blob/a36f42a5b181d0c9cc84174e8eb241b11eeabc0f/emacs/merlin.el#L177C12-L177C39) 
- where the value 'diff' uses a different window if the destination is 
in an file different from the current one.

With the attached patch the customization looks a bit noisier though:

   (setq xref-navigation-display-window-action
         '(display-buffer-reuse-window))

^ This makes it try to reuse an existing window and fall back to 
pop-to-window, but the effect is similar to what's described above.

Comments welcome.