Set to true for export, false for import. If export true, an
XMLPropertyMapEntry with mbImportOnly==true is not added to the
mappings. This to be able to have more than one mappings for import
(for example a current extension namespace and the future namespace
proposed to the ODF-TC, or corrected typos in element or attribute
names), but map only to one entry on export, of course.

Change-Id: Ia01ea949c88eda2f8a6c10f51c59e35e7abdcaf3