Improve room past events loading

- Trigger when room is shown if there's not enough messages to fill the
  list height

- Trigger whenever user is scrolling before a certain point, instead of
  when dragging is released/scrolling stopped and the top edge is hit

- Prevent multiple load requests at same time

- Keep a set of fully loaded rooms, don't request anymore history
  if a room is fully loaded

harmonyqml/backend/signal_manager.py

@@ -93,6 +93,9 @@ class SignalManager(QObject):
             self, _: Client, room_id: str, token: str
         ) -> None:
 
+        if self.backend.past_tokens[room_id] == token:
+            self.backend.fully_loaded_rooms.add(room_id)
+
         self.backend.past_tokens[room_id] = token