Python: Use kwargs passed to the instance constructors

And set properties, like in JavaScript.

api/python/slint/__init__.py

@@ -98,6 +98,9 @@ def _build_class(compdef):
                     self.__instance__.set_callback(
                         name, mk_callback(self, value))
 
+        for prop, val in kwargs.items():
+            setattr(self, prop, val)
+
     properties_and_callbacks = {
         "__init__": cls_init
     }

api/python/tests/test_load_file.py

@@ -38,3 +38,18 @@ def test_load_file_wrapper():
     assert instance.MyGlobal.global_prop == "This is global"
 
     del instance
+
+
+def test_constructor_kwargs():
+    module = load_file(os.path.join(os.path.dirname(
+        __spec__.origin), "test_load_file.slint"), quiet=False)
+
+    def early_say_hello(arg):
+        return "early:" + arg
+
+    instance = module.App(hello="Set early", say_hello=early_say_hello)
+
+    assert instance.hello == "Set early"
+    assert instance.invoke_say_hello("test") == "early:test"
+
+    del instance