BuildManager::cancel() needs to block until the step is canceled

Task-number: QTCREATORBUG-10256

Change-Id: I8e761ac072954eb1802b9b4eec415f48414b7990
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>