Qualified name wrapper.
This class can be used to wrap a QName attribute value in order to get
proper namespace handing on output.
*text_or_uri* is a string containing the QName value either in the form
{uri}local, or if the tag argument is given, the URI part of a QName.
*tag* is an optional argument which if given, will make the first
argument (text_or_uri) be interpreted as a URI, and this argument (tag)
be interpreted as a local name.