qbs build: Introduce convenience item QbsProduct.

As base product for QbsApp, QbsLibrary etc. Has little functionality
at the moment, but will get more soon.

Change-Id: I29ac78db2ff1ba109dd09a55e7593b2beeaba741
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>