Todo: Fix thread safety issue in CppTodoItemScanner

Replace the not thread safe Project::files() with the thread safe
CppModelManager::projectPart() to find out whether a file belongs to a
project or not.

Task-number: QTCREATORBUG-12556
Change-Id: Icf443b5b31c818fc15738aa365562db12439d8d7
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>