Add 'handle_once' option to RegisterEventHandler

This allows the user to configure "one shot" events via launch actions.

Closes #111.

launch/launch/actions/register_event_handler.py

@@ -32,10 +32,11 @@ class RegisterEventHandler(Action):
     place.
     """
 
-    def __init__(self, event_handler: EventHandler, **kwargs) -> None:
+    def __init__(self, event_handler: EventHandler, handle_once: bool = False, **kwargs) -> None:
         """Constructor."""
         super().__init__(**kwargs)
         self.__event_handler = event_handler
+        self.__event_handler.handle_once = handle_once
 
     @property
     def event_handler(self) -> EventHandler: