With this, it is possible to import part of the document as RDF
statements later when SwFltControlStack::SetAttrInDoc() gets an
SwFltRDFMark.

Previously SfxBaseModel member functions like
getMetadataGraphsWithType() and addMetadataFile() failed, as they tried
to find the already imported document in UCB, which failed, as the
import was still in progress. To prevent that, do the same as the ODT
import in XMLReader::Read(), part "RDF metadata".

One trick here is the call to
comphelper::OStorageHelper::GetTemporaryStorage(), which gives an empty
storage. Ideally a wrapper class would be needed that works on a
SotStorage, but implements embed::XStorage, but that would be only used
to find that the real storage doesn't provide a manifest.rdf stream,
which is always the case. So instead of writing such a wrapper, just
give loadMetadataFromStorage() an empty storage, which will have the
same result without writing lots of dead code.

Change-Id: Id1897838b1477eee0489b706d51cb6f59898877b