Cocoa color dialog: Make sure hide() exits the modal session

Change-Id: I708011f3a8ce507e6ade98bf99ad0f5d7b160751
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
QSizePolicy - add retainSizeWhenHidden

Sometimes it is nice that hiding a widget does not affect the
layout. This patch makes that possible by allowing hidden
widgets to take up space.

Change-Id: Ifbc1cdee0e112950acc025919b98199ea9558db7
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>