Add a (non-gui) setting GeneratorOverride that, if set, is used
for odt/docx/pdf generator string identifying the application that
has written it instead of the normal productname etc. string.
Change-Id: Ibe12a063e7e6b5a09c69cdf66cba073793650d62
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/132495
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>