Add xref forward history (bug#38797)

Add xref forward history (bug#38797)

Make it possible to go forward as well as back in the Xref history.
The new `xref-go-forward` is bound to `C-M-,`.

* lisp/progmodes/etags.el (find-tag-marker-ring-length): Update.
(tags-location-ring-length): New.
(find-tag-marker-ring): Keep as dummy.
(tags-location-ring, tags-reset-tags-tables):
Use `tags-location-ring-length` instead of `xref-marker-ring-length`.
* lisp/progmodes/xref.el (xref-marker-ring-length, xref-marker-ring)
(xref-set-marker-ring-length):
Make obsolete.
(xref--history, xref-go-forward, xref-forward-history-empty-p): New.
(xref-push-marker-stack, xref-pop-marker-stack)
(xref-clear-marker-stack, xref-marker-stack-empty-p):
Use `xref--history`.
* lisp/menu-bar.el (menu-bar-goto-menu): Add Forward entry.
* doc/emacs/maintaining.texi (Looking Up Identifiers): Document.
* etc/NEWS: Announce.