Widget Designer: Fix modified state indicator

It failed to update internal state when saving.

Change-Id: I0b260e55dc50fa39958914a0d4fa3f5e73fe1c27
Task-number: QTCREATORBUG-12279
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>

src/plugins/designer/formwindowfile.cpp

@@ -96,7 +96,7 @@ bool FormWindowFile::save(QString *errorString, const QString &name, bool autoSa
 
     m_formWindow->setDirty(false);
     setFilePath(fi.absoluteFilePath());
-    emit changed();
+    updateIsModified();
 
     return true;
 }