Let MetafileXmlDump::dumpAndParse() take a const GDIMetaFile

So the const_cast<> in CppunitTest_sw_uiwriter can go away.

Change-Id: Id7d4234154cd6bed43cb5270b50208061038dcf7
Factor out SwFmtCntnt::dumpAsXml() from nodedump

Change-Id: I6adcaa8534efefa474d489125d3fb95f150e3e89