Limit NatNum12 conversion only for the selected parts of the
date format (this bug – double calls of getNumberText – was hidden
by the space prefix " " and empty return values at the first calls,
resulting unchanged dates yet).

New prefixes: "capitalize", "upper" and "title" to handle optional
capitalization. (In Calc, it was not possible to format the result of
NatNum formatting, but some languages often need capitalization
or title case to format numbers and currencies.)

Thanks code clean up using enum WhichCasing to Eike Rathke.

Change-Id: I5fceb784930e6bc6d376116f5a42ad49cd248a54
Reviewed-on: https://gerrit.libreoffice.org/55681
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>