[OS X]: respect icon visibility preference in system tray menus

Check the `IconVisibleInMenu` switch so that systray menu items only
show an icon if the application/user has configured things that way,
and not otherwise.

Task-number: QTBUG-41348
Change-Id: Ibb7db326fd598ca78cb95c815b5e165de2babeda
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>