Make the argument to dispatch_event_with_target non-optional.

The name of the method makes it clear it's supposed to be used with a target
override, so we might as well enforce that.
auto merge of #4443 : Ms2ger/servo/dispatch_event_with_target, r=Manishearth

The name of the method makes it clear it's supposed to be used with a target
override, so we might as well enforce that.