This moves BackGraphicURL, HeaderGraphicURL, FooterGraphicURL and
ParaBackGraphicURL properties to BackGraphic, HeaderBackGraphic,
FooterBackGraphic and ParaBackGraphic. With this the property type
changes from String to XGraphic.

This change also fixes a bunch of test failures, changes the tests
to use the new properties and the correct type, changes the import
and export filters like xmloff and oox, to make the tests happy.

Change-Id: Ie66097514203c6dc36ab27420faf265322e9279e
Reviewed-on: https://gerrit.libreoffice.org/50760Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>