QAbstractItemModel trait?

[LeonMatthesKDAB]

Add a trait for QAbstractItemModel subclasses to use, similar to our QtObject trait (see #562 ).

Would look something like this

trait AbstractItemModel {
    fn as_abstract_item_model() -> *mut QAbstractItemModel;

    fn set_data(&self, index: &QModelIndex, value: &QVariant, role: i32) {
         as_abstract_item_model().set_data(/*...*/); // calls default implementation, which does nothing.
    }
    fn data(&self, index: &QModelIndex); // required method
}