Another refactor of the OOP HTML/XML generation code. (Sorry.)

Another refactor of the OOP HTML/XML generation code.  (Sorry.)

New class XmlContent for a collection of XML content (without a tag).
(You should use this instead of an array, for representing a
sequence of XML elements.)  (The function HTML() is a shortcut
constructor, so where you would have done:
  $html = array(HTML::h2("Header),
                HTML::p("Paragraph"));
you should now do
  $html = HTML(HTML::h2("Header),
               HTML::p("Paragraph"));

And, instead of
  $html[] = HTML::p("Another par.");
do
  $html->pushContent(HTML::p("Another par."));

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@1668 96ab9672-09ca-45d6-a79d-3d69d39ca109