Make window choice in xref commands configurable

Make window choice in xref commands configurable

Previously, it wasn't easy to tell xref.el commands like
xref-find-definitions or xref-find-definitions-other-window how to
choose a window for the *xref* buffer or how to find windows for
displaying the results after choosing a candidate.  This patch makes
that task easier, but keeps the current behaviour intact.

Co-authored-by: João Távora <joaotavora@gmail.com>
* lisp/progmodes/xref.el (xref--show-pos-in-buf): Simplify.