Some cleanup in the C++ preprocessor.

Introduced pp-scanner.cpp, renamed `pp' to `Preprocessor' and removed useless #includes.

src/plugins/cpptools/cppmodelmanager.cpp

@@ -144,7 +144,7 @@ private:
     QPointer<CppModelManager> m_modelManager;
     Snapshot m_snapshot;
     Environment env;
-    pp m_proc;
+    Preprocessor m_proc;
     QStringList m_includePaths;
     QStringList m_systemIncludePaths;
     QMap<QString, QByteArray> m_workingCopy;