We relied on the CppModelManager to tell us whether a project was reparsed
after a kit change. While this worked, it was not guaranteed that the project
is really finished (and ready for e.g. building) after pushing new ProjectInfos
to the CppModelManager.

Rely on the projects telling when they are finished with parsing. This is more
accurate and future-proof.

The introduced signals in Project and SessionManager are (at the moment)
only for tests.

Change-Id: I1b368ec4585ffa8755eb28fac6d187cce31243ee
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>