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>