TextEditor: Update actions after toggling read only mode.

Task-number: QTCREATORBUG-9188

Change-Id: I3448645397e7a39487f2e961728ebef924eec7f6
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/texteditor/basetexteditor.h

@@ -325,6 +325,7 @@ public slots:
 signals:
     void changed();
     void assistFinished();
+    void readOnlyChanged();
 
     // ITextEditor
     void contentsChanged();