CppTools: Start to use editor document

This is only a small refactoring step towards a editor-free
CppEditorSupport.

Left are m_editor references that need to be addressed.

No functional change.

Change-Id: I9fc7b0abfe88f5b9a6ce3f57a5c4a922ae953ddf
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cpptoolseditorsupport.h

@@ -208,6 +208,7 @@ private:
 private:
     Internal::CppModelManager *m_modelManager;
     QPointer<TextEditor::BaseTextEditor> m_textEditor;
+    TextEditor::BaseTextDocument *m_editorDocument;
     QTimer *m_updateDocumentTimer;
     int m_updateDocumentInterval;
     unsigned m_revision;