Remove menu role conflict for preferences dialog

That conflict would result on the preferences dialog not being
accessible on Mac.

Change-Id: Ibd2e370c3e29f672d0df847ecf90ea1022a20fdc
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>