Add new function 'dom-inner-text'

Add new function 'dom-inner-text'

This replaces 'dom-text' and 'dom-texts', and is both more correct and
more efficient than them.

* lisp/dom.el (dom-text, dom-texts): Make obsolete in favor of...
(dom-inner-text--1, dom-inner-text): ... these new functions.  Update
callers.

* doc/lispref/text.texi (Document Object Model): Update documentation to
refer to 'dom-inner-text'.

* etc/NEWS: Announce this change.

(cherry picked from commit 2e53c7d08ba0de2468ec25cf9a9d7079604f9409)