They do what you expect them to do, that is build only a subtree.
The implementation is a little bit strange. We temporarly set the sub
node to be built on the BuildConfiguration, enqueue that
BuildConfiguration (which calls BuildStep::init()), and then reset that
temporary value.

A more general way would be to have the ability to extend
BuildConfiguration::buildProject() in a way that additional data can be
passed to the BuildSteps.

Task-Nr: QTCREATOR-5
Task-Nr: QTCREATORBUG-44