CppEditor: Fix "Rename Symbol Under Cursor" in new split

The slot CPPEditorWidget::onContentsChanged() was still connected to the
initial QTextDocument of BaseTextEditorWidgetPrivate and not to the via
BaseTextEditorWidget::duplicateFrom() updated QTextDocument.

This fixes the visual appearance when renaming. The actual renaming
is/was not affected.

Task-number: QTCREATORBUG-9651
Change-Id: Id26dc11627c253bbf89904be3f3df21a45041d01
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>