CppTools: Use namespace CppTools consistently

There were quite a few classes using CPlusPlus namespace in the
CppTools plugin. Rename them and do some other small namespace
related coding style fixups.

Change-Id: I093fc1f3fc394fd9923e3f18d5f66522e288f21d
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cppeditor/cppquickfix.h

@@ -34,10 +34,7 @@
 
 #include <texteditor/quickfix.h>
 
-namespace CPlusPlus {
-class CppModelManagerInterface;
-class Snapshot;
-}
+namespace CPlusPlus { class Snapshot; }
 
 namespace CppEditor {
 namespace Internal { class CppQuickFixAssistInterface; }