HeaderView: it's not possible to edit a cell in a HeaderView

It's currently not possible to edit a cell in a HeaderView (using e.g TableView.edit(), or clicking on a cell). The reason being that TableView requires the model to return Qt::ItemIsEditable from QAbstractItemModel::flags(index). But the application developer doesn't have access to the model used by HeaderView, because it's an internal proxy model that fetches the headerData() from the model assigned to the syncView.