Use event handlers' return value for propagation

Fixes ee.event.propagationStopped_ not being set.

goog.events.EventTarget.dispatchEventInternal_ makes ee part of a larger goog.events.Event object, so if an event handler calls e.stopPropagation() then propagationStopped_ is set on the goog.events.Event object and ee doesn't have access to it.  (dispatchEventInternal_: https://github.com/jarib/google-closure-library/blob/master/closure/goog/events/eventtarget.js#L490-493, the e being passed in is a lime.events.Event).

Another solution is to make lime.events.Event a subclass of goog.events.Event, wasn't sure which way you'd want to go.