Add xref-based replacements for Dired search commands

Add xref-based replacements for Dired search commands

* lisp/dired-aux.el (dired-do-find-regexp)
(dired-do-find-regexp-and-replace): New commands.
http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00864.html

* lisp/dired.el (dired-mode-map): Change bindings for `A' and
`Q' to the new commands.

* lisp/progmodes/xref.el (xref-query-replace)
(xref-collect-matches): Add progress reporters.
(xref--find-ignores-arguments): Return nil for zero ignores.
(xref--show-xrefs): Add an optional argument.
(xref-collect-matches): Drop the assert.  'find' accepts a
regular file in place of directory argument, too.