Start a fresh DFXML schema

This will simplify the structure of the fileobject schema.
(In progress) Grafting schema data from fileobject.xsd to dfxml.xsd
Add local copy of Dublin Core metadata so xmllint will validate

<http://www.dublincore.org/schemas/xmls/qdc/2008/02/11/dc.xsd>
(Untested) Finish extracting DFXML schema components used in Fiwalk