Make it possible to indicate that a widget with Qt::Sheet set can be made resizable

Make it possible to indicate that a widget with Qt::Sheet set can be made resizable. On Mac a sheet can be resizable as it depends on the context but currently there is no way to make one resizable in the obvious sense. It can be done by setting the Qt::WindowMaximizeHint as a window flag but it should be possible in a more obvious way at least.