Add new queuing features to @futurize

- max_instances renamed to max_running

- consider_args parameter:
  if True, $max_running of this function with the same arguments can be
  running, else:
  $max_running of this function, no matter the arguments, can be running

- discard_if_max_running:
  if True and there are already the maximum possible number of running
  functions running, cancel this task (previous default behavior), else:
  Wait for a spot to be free before running

harmonyqml/backend/backend.py

@@ -60,7 +60,7 @@ class Backend(QObject):
 
     @pyqtSlot(str, result="QVariant")
     @pyqtSlot(str, bool, result="QVariant")
-    @futurize()
+    @futurize(max_running=1, consider_args=True)
     def getUserDisplayName(self, user_id: str, can_block: bool = True) -> str:
         if user_id in self._queried_displaynames:
             return self._queried_displaynames[user_id]