If minimizer is used on current presentation without making
a copy and it has local modification we should suggest user
to save it before, since not all minimization opearions could
be reverted later.

Change-Id: I12600c9a818698418f3ec957d43b13c6044609d2
Reviewed-on: https://gerrit.libreoffice.org/64261
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>