in preparation for "scoped UNO enums".

This is a little hacky: In order to limit the scope of this change,
the templated SvXMLEnumMapEntry struct actually has a fixed size field,
and we cast it to SvXMLEnumMapEntry<sal_uInt16>* in various
places, to avoid carrying the type param around.

Change-Id: Idfbc5561303c557598dd5564b7a7259ae5261d83
Reviewed-on: https://gerrit.libreoffice.org/34987Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>