Python: Initial support for implementing models in Python

This provides a Model base class in Python and sub-classes of that
can be set as data models in slint.

The ListModel is provided as basic sub-class operating on a list() and
allowing mutation and notifying the view on the Slint side.

Similarly, an array declared in Slint is exposed as an object to Python
that looks like a Model.

Both support the sequence protocol.

Fixes #4135

api/python/lib.rs

@@ -6,6 +6,7 @@ mod interpreter;
 use interpreter::{ComponentCompiler, PyDiagnostic, PyDiagnosticLevel, PyValueType};
 mod brush;
 mod errors;
+mod models;
 mod timer;
 mod value;
 
@@ -38,6 +39,7 @@ fn slint(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
     m.add_class::<timer::PyTimer>()?;
     m.add_class::<brush::PyColor>()?;
     m.add_class::<brush::PyBrush>()?;
+    m.add_class::<models::PyModelBase>()?;
     m.add_function(wrap_pyfunction!(run_event_loop, m)?)?;
     m.add_function(wrap_pyfunction!(quit_event_loop, m)?)?;