Allow users to select installer style

Add a "WizardStyle" tag to config.xml that takes values "Modern", "Classic", "Aero" or "Mac" to force the style.

Change-Id: Ibd6c329a6b159c202fa7a7aa535b36e00c544079
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>