Better handling of HTML5 drop events

I noticed, that html5 drop events are always consumed by the world. That's why morphs, which listen on this event, are ignored.
This first fix, checks if there are underlying morphs which listen on this event. Only, if there are no morphs, the event is consumed.
The direct check whether ea.eventHandler.dispatchTable["drop"] exists is not very nice. Maybe somebody has a better idea. Let me know, what you think.