Add Container.__getitem__ to allow slicing

src/linkahead/common/models.py

@@ -3001,6 +3001,10 @@ class Container(list):
     def __repr__(self):
         return xml2str(self.to_xml())
 
+    def __getitem__(self, key):
+        # Construct new Container from list slice
+        return Container().extend(super().__getitem__(key))
+
     @staticmethod
     def from_xml(xml_str):
         """Creates a Container from the given xml string.