modify sax::Converter::convertMeasure to use sal_Int64 instead of BigInt:
  should be sufficient, since the largest number is SAL_INT32_MAX * 10^7.
 remove duplicate methods from SvXMLUnitConverter:
  convertMeasurePx, convertMeasure (static variants)
 remove entirely duplicative class SvXMLExportHelper:
  GetConversionFactor, GetUnitFromString, AddLength
 change SvXMLUnitConverter interface from MapUnit to css::util::MeasureUnit.
 change SvXMLExport constructor params from MapUnit to css::util::MeasureUnit.
 rename some methods to turn compiler into merge conflict detector :)