CppEditor: Partially move to new editor setup scheme

This needs more reorganization. For now, use a method to
add cpp editor support directly without relying on a fully
set up editor/widget/document combo.

Change-Id: Id8631c75ccd209f418faef31f8b8a4c71e53d87d
Reviewed-by: Christian Stenger <christian.stenger@digia.com>