When the object is edited in the UI, the m_xClient is set to a
SfxInPlaceClient and the DocumentBaseURL is retrieved from it.  But if
the object is not edited, it will be loaded during export via the API
and without a m_xClient; in this case the DocumentBaseURL must have been
set previously to be available during import.

There appears to be no way to get the URL of the document via the API
while it is being imported; SfxBaseModel's m_sURL is unfortunately only
initialized from SfxObjectShell::FinishedLoading().

During ODF import, the SvXMLEmbeddedObjectHelper creates the
embedded object, so let's make it pass in the parent's BaseURL.

The "DefaultParentBaseURL" parameter already exists but was unused
previously.

Change-Id: I3d1ed29b3a2c0e77ec606a1d09f7bc07e7860733