Add the settings directory to the project setup parameters.

This way, the settings become available for use in the qbs library,
which means it can, for instance, expand the build configuration itself
instead of putting this burden onto the API callers.

Change-Id: I09e6067990ac2b7c2238951f9759fe52dc51b6f9
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>