BuildConfiugration::subConfigWidge() Provide a default implementation

Which simply returns a BuildEnvironmentWidget. A long time ago
BuildConfigurations had no environment, nowdays they do. So it makes
sense for all BuildConfigurations to have the BuildEnvironmentWidget.

Change-Id: I824c45df79a0dcd2b624bf67a4730fb5dab098bc
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>