QbsProjectManager: Use the qbs::Profile abstraction.

No need to poke around in the settings directly.

Change-Id: I689039a0cb0afd2e3a4c32b2108755d79c8fb4b6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>