C++: Moved completion/highlighting into the model manager.

This way the editor does not need to know all the details of
instantiating or maintaining classes for highlighting and/or completion,
it can just ask the model manager. The change also enables different
highlighting- or completion-engines without changes to the cppeditor.

Change-Id: I8000d9d9fe446b292defddb2295493cf77d0f14a
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>