don't stack up "no highlighting information" info bars

don't try to add another bar one if there is one already

Task-number: QTCREATORBUG-4951
Change-Id: Ia7282a5ee96d3b6b08b3f66c3bc162b1c4a5a015
Reviewed-on: http://codereview.qt.nokia.com/32
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>

src/plugins/texteditor/basetextdocument.h

@@ -103,6 +103,9 @@ public:
     bool reload(QString *errorString, QTextCodec *codec);
     void cleanWhitespace(const QTextCursor &cursor);
 
+    bool hasHighlightWarning() const;
+    void setHighlightWarning(bool has);
+
 signals:
     void titleChanged(QString title);