CppEditor: Clean up CppEditor/CppEditorWidget

* Excess includes and forward declarations
* Access protection levels
* QTC_OVERRIDE
* Remove declared but not implemented/used functions.
* Remove unused class

Change-Id: Ia19ddddccd9adde827a7875b466fde5ca4d96e7d
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>