The XDocuments representing the DOM of an OOXML's theme document is
now stored as the PropertyValue "OOXTheme" into the "InteropGraBag"
property of the SwXTextDocument.

Added the importSubStream() method to the
writerfilter::ooxml::OOXMLDocumentImpl class in order to create the
XDocument for the theme XML fragment stream and invoked it during the
resolve() method.

Modified the writerfilter::ooxml::OOXMLDocument abstract class to be
able to hold the OOXML's theme XDocument. Added the set/getTheme()
methods to store and obtain the XDocument directly.

Modified the WriterFilter::filter() method to store the OOXML's theme
XDocument in the "InteropGrabBag" property of the TextDocument service
SwXTextDocument implementation.

Updated the UTs accordingly.

Change-Id: I3960590fcf4856caf770908b983ffb6898f061c0
Reviewed-on: https://gerrit.libreoffice.org/6002Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>