Introduce ILogSink::printWarning().

To be called when warning about some issue in a specific file. Can be
overridden to e.g. display the problematic location in an IDE.
This replaces the BuildJob::reportWarning() signal.

Task-number: QBS-284
Change-Id: Ib5c14f6793c5b59144bfd9ff7ae11ae94416b2b2
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>