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/slint/__init__.py

@@ -14,3 +14,4 @@ def load_file(path):
 Image = native.PyImage
 Color = native.PyColor
 Brush = native.PyBrush
+Model = native.PyModelBase

api/python/slint/models.py

@@ -0,0 +1,71 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.1 OR LicenseRef-Slint-commercial
+
+from . import slint as native
+
+
+class Model(native.PyModelBase):
+    def __new__(cls, *args):
+        return super().__new__(cls)
+
+    def __init__(self, lst=None):
+        self.init_self(self)
+
+    def __len__(self):
+        return self.row_count()
+
+    def __getitem__(self, index):
+        return self.row_data(index)
+
+    def __setitem__(self, index, value):
+        self.set_row_data(index, value)
+
+    def __iter__(self):
+        return ModelIterator(self)
+
+
+class ListModel(Model):
+    def __init__(self, lst=None):
+        super().__init__()
+        self.list = lst or []
+
+    def row_count(self):
+        return len(self.list)
+
+    def row_data(self, row):
+        return self.list[row]
+
+    def set_row_data(self, row, data):
+        self.list[row] = data
+        super().notify_row_changed(row)
+
+    def __delitem__(self, key):
+        if isinstance(key, slice):
+            start, stop, step = key.indices(len(self.list))
+            del self.list[key]
+            count = len(range(start, stop, step))
+            super().notify_row_removed(start, count)
+        else:
+            del self.list[key]
+            super().notify_row_removed(key, 1)
+
+    def append(self, value):
+        index = len(self.list)
+        self.list.append(value)
+        super().notify_row_added(index, 1)
+
+
+class ModelIterator:
+    def __init__(self, model):
+        self.model = model
+        self.index = 0
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.index >= self.model.row_count():
+            raise StopIteration()
+        index = self.index
+        self.index += 1
+        return self.model.row_data(index)