Do not create rdf objects if the xml:id doesn't start with rdfid-

Following the deprecation of draw:id and text:id, elements that used to
just save draw:id and text:id now also save these id's as xml:id.

Calligra's rdf support thinks that every xml:id is a reference to some
rdf metadata. This breaks because there _is_ no rdf data associated.

Until I've found a proper solution to shared xml:id's between various
parts of calligra, check whether the xml:id starts with rdfid- and only
then load the metadata.

Yes, this probably means we can no longer load documents with rdf saved
by other applications like abiword. I'm working on a "proper" solution.