- fix interface of sax::Converter to allow passing time zones
  (rename the parsing methods while at it to reduce pointless overloading)
- SfxDocumentMetaData supports DateWithTimezone and DateTimeWithTimezone
  in user-defined properties
- add some ugly hacks to SfxCustomPropertiesPage to preserve existing
  time zones (which are not displayed in UI currently)

Change-Id: Ice94112b9d79c285f80b5beda15f0ace91db97f3