in the whole createElement vs. createElementNS mess.

The first part makes the testcase in #230497 pass (though that bug 
also uncovers an another issue)

Now that Gecko consistently returns xhtml namespaceURI for html elements, 
we can also get rid of some compatibility mode code.

CCBUG:230497


svn path=/trunk/KDE/kdelibs/; revision=1102863