AddOnce Listener will be cleared before dispatching

Instead of calling OnceDelegates and afterwards clearing the OnceDelegates list, it will copy them into a temporary variable, clear the OnceListener class event before calling the delegates via the temporary variable. 

This helps in cases where the handle or a subsequent following method might add another AddOnce-Listener to the same signal, which won't be called when the signal gets dispatched again (as the list is cleared afterwards till now).