Keep the xref buffer visible until the user quits it explicitly

Keep the xref buffer visible until the user quits it explicitly

* lisp/progmodes/xref.el (xref--pop-to-location):
Rename WINDOW argument to ACTION.
(xref--with-dedicated-window): New macro.
(xref--show-pos-in-buf): Rename from `xref--display-position'.
Add and handle new argument, SELECTED.  Use the above macro.
(xref--show-location): Add SELECTED argument.
(xref-show-location-at-point): Make an effort to avoid the
original window when showing the location.
(xref-goto-xref): Don't quit the xref window (bug#20487 and
http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg01133.html).
(xref--query-replace-1): Use xref--with-dedicated-window as well.
(xref--next-error-function): Call xref--show-location instead of
xref--pop-to-location.
(xref--show-xrefs): Rename WINDOW argument to DISPLAY-ACTION.
Only pass that value to xref--pop-to-location.  Pass the current
selected window to xref-show-xrefs-function as the `window'
property.
(xref--find-xrefs, xref--find-definitions): Rename WINDOW argument
to DISPLAY-ACTION as well.