Honor window-switching intents in xref-find-definitions (bug#28814)

Honor window-switching intents in xref-find-definitions (bug#28814)

When there is more than one xref to jump to, and an *xref* window
appears to help the user choose, the original intent to open a
definition in another window or frame is remembered when the choice to
go to or show a reference is finally made.

* lisp/progmodes/xref.el (xref--show-pos-in-buf): Rewrite.
(xref--original-window-intent): New variable.
(xref--original-window): Rename from xref--window and move up
here for clarity.
(xref--show-pos-in-buf): Rewrite.  Don't take SELECT arg here.
(xref--show-location): Handle window selection decision here.
(xref--window): Rename to xref--original-window.
(xref-show-location-at-point): Don't attempt window management here.
(xref--show-xrefs): Ensure display-action intent is saved.