sw: fix DOC/DOCX/RTF export of TextFrame background color

Change-Id: I8b7c23af7548a0eb073e046eedfbabfb9f78fafe
sw: put getSvx* and setSvx* to a namespace, now that these are public

Change-Id: Iea2f6af83382608e4a36dd2e5803cbb936b78058