So far the simplified project tree was a flat list of files for qbs
projects. With this change it is a flat list of Products, each
containing a flat list of all files referrenced.

Change-Id: If53e80ca69f595df245a20af9f9ced1121cf30af
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>