Add support for mapping image properties

This exposes a slint.Image class, which has a load_from_path class
method as well as size/width/height properties.

cc #4202

api/python/slint/__init__.py

@@ -8,3 +8,5 @@ def load_file(path):
     compdef = compiler.build_from_path(path)
     instance = compdef.create()
     return instance    
+
+Image = native.PyImage