Move transparence tab code out from Area tab code.

Change-Id: Ia7276aa3b1c438b6c8cff89ab5ca3752ff6bcc0e
Reviewed-on: https://gerrit.libreoffice.org/24368Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>