Qbs: Show products in simplified project tree

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>
Project: Do not lose project file path information

... when opening a project that is no longer found on disk.

Task-number: QTCREATORBUG-10703
Change-Id: I5163c85469d354d2cf48959dcae6303c69392b8f
Reviewed-by: Daniel Teske <daniel.teske@digia.com>