Don't remap `next-line' and `previous-line' in xref keymap

Don't remap `next-line' and `previous-line' in xref keymap

* lisp/progmodes/xref.el (xref--window-configuration): New variable.
(xref-show-location-at-point): New command.
(xref--restore-window-configuration): New function.
(xref-next-line, xref-prev-line): Delegate to
`xref-show-location-at-point'.
(xref--location-at-point): Don't signal the error.
(xref-goto-xref): Do that here instead.
(xref--xref-buffer-mode): Add `xref--restore-window-configuration'
to `pre-command-hook'.
(xref--xref-buffer-mode-map): Don't remap `next-line' and
`previous-line'.  Additionally bind `xref-next-line' and
`xref-prev-line' to `n' and `p' respectively.  Bind
`xref-show-location-at-point' to `C-o'.