Python: Improve Struct mapping

When reading, create the local equivalent of a dataclass, so that access
doesn't require ["foo"] key syntax.

Also implement the copy protocol, so that we can safely make clones of
the references returned by the ListModel.

api/python/lib.rs

@@ -41,6 +41,7 @@ fn slint(_py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<brush::PyColor>()?;
     m.add_class::<brush::PyBrush>()?;
     m.add_class::<models::PyModelBase>()?;
+    m.add_class::<value::PyStruct>()?;
     m.add_function(wrap_pyfunction!(run_event_loop, m)?)?;
     m.add_function(wrap_pyfunction!(quit_event_loop, m)?)?;