ProxyModel::rowCount()'s parent parameter should have a default value.

QAbstractItemModel::rowCount() has a default value for the parent parameter:
http://doc.trolltech.com/4.6/qabstractitemmodel.html#rowCount
but the ProxyModel::rowCount() override does not have the default parameter value:
http://doc.trolltech.com/4.6/qproxymodel.html#rowCount

This makes it slightly harder to use the QAbstractItemModel API polymorphically.

I know that default parameter values are awkward with virtual methods, but I see that other derived classes do keep the default parameter values, so fixing this would make Qt more consistent.