Support bookmarking Xref results buffers

Support bookmarking Xref results buffers

* lisp/progmodes/xref.el (bookmark-make-record-default)
(bookmark-make-record, bookmark-prop-get)
(bookmark-handle-bookmark, bookmark-get-rear-context-string)
(bookmark-get-front-context-string): Declare functions.
(xref-backend-context, xref-backend-restore): New generic functions.
(xref--backend, xref--identifier, xref--kind)
(xref--original-buffer, xref--original-point): New local variables.
(xref--show-common-initialize): Set them in Xref results buffer.
(xref-default-bookmark-name-format): New user option.
(xref-bookmark-make-record, xref-bookmark-jump): New functions.
(xref--xref-buffer-mode): Set 'bookmark-make-record-function'.
(xref-fetcher-alist): New variable.
(xref--show-xref-buffer, xref-show-definitions-buffer)
(xref-show-definitions-buffer-at-bottom): Use it.
(xref--read-identifier): Improve error message.
(xref-make-fetcher): Extract from...
(xref--create-fetcher): ...here.

* doc/emacs/maintaining.texi (Xref Commands): Document new feature.

* etc/NEWS: Announce new feature and Xref generic functions.