Fix marking files modified externally as modified

It replaces 59c90e00c1
and d0c537ca75

Task-number: QTCREATORBUG-17048
Change-Id: Ief4b1b72f2e5e7b1711be05d4ea8c03bbbf48fdf
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/texteditor/textdocument.h

@@ -151,6 +151,7 @@ private:
                         bool reload);
     void cleanWhitespace(QTextCursor &cursor, bool cleanIndentation, bool inEntireDocument);
     void ensureFinalNewLine(QTextCursor &cursor);
+    void modificationChanged(bool modified);
 
     TextDocumentPrivate *d;
 };