QtQuick.Dialogs.ColorDialog: size is sane on iOS

480 is too big in the new-fangled "logical pixels", and the dynamic
size adjustment strategy also doesn't work well because the sliders
are so tall.

Change-Id: I21080cf7bd36e2d29d8be08320dacc7a0e354a06
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>