CppTools: Clean up CppModelManager(Interface)

* Const correctness
* Better variable names
* Sort order of includes and forward declarations
* Comment fixes
* Cosmetic/Whitespace changes

Change-Id: Ieb137266ac7eddedb00c37f8b2cc8677d67e4bdd
Reviewed-by: David Schulz <david.schulz@digia.com>