* lisp/progmodes/xref.el (xref--button-map): New variable.
(xref--mouse-2): New command.
(xref--insert-xrefs): Add `mouse-face' and `keymap' properties to
the inserted references.
* 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'.
* lisp/emacs-lisp/find-func.el (find-function-library): New function,
extracted from `find-function-noselect'.
* lisp/progmodes/elisp-mode.el (elisp--identifier-location): Fold back
into `elisp--company-location'.
(elisp--identifier-completion-table): Rename to
`elisp--identifier-completion-table', and do not include just any
symbols with a property list.
(elisp-completion-at-point): Revert the 2014-12-25 change.
(elisp--xref-identifier-file): New function.
(elisp--xref-find-definitions): Use it.
* lisp/progmodes/xref.el (xref-elisp-location): New class.
(xref-make-elisp-location): New function.
(xref-location-marker): New implementation.
* lisp/progmodes/elisp-mode.el (elisp--xref-find-definitions):
Filter out nil results.
(elisp--xref-find-apropos): New function.
(elisp-xref-find): Use it.
* lisp/progmodes/xref.el (xref--show-xrefs): Use `user-error'.
Move autoloaded bindings for `M-.', `M-,', `C-x 4 .' and
`C-x 5 .' from etags.el to xref.el.
* progmodes/xref.el: New file.
* progmodes/elisp-mode.el (elisp--identifier-types): New variable.
(elisp--identifier-location): New function, extracted from
`elisp--company-location'.
(elisp--company-location): Use it.
(elisp--identifier-completion-table): New variable.
(elisp-completion-at-point): Use it.
(emacs-lisp-mode): Set the local values of `xref-find-function'
and `xref-identifier-completion-table-function'.
(elisp-xref-find, elisp--xref-find-definitions)
(elisp--xref-identifier-completion-table): New functions.
* progmodes/etags.el (find-tag-marker-ring): Mark obsolete in
favor of `xref--marker-ring'.
(tags-lazy-completion-table): Autoload.
(tags-reset-tags-tables): Use `xref-clear-marker-stack'.
(find-tag-noselect): Use `xref-push-marker-stack'.
(pop-tag-mark): Make an alias for `xref-pop-marker-stack'.
(etags--xref-limit): New constant.
(etags-xref-find, etags--xref-find-definitions): New functions.
