Added EventDispatcher#remove method
Renamed HasHandlers into HasEventHandlers and put into gui.event root
* New EventListener, because an EventHandler had previously two tasks,
  declare a `onEvent` method and deligate a event of this type to the
  `onEvent` method, now the EventHandler deligates an event to the
  EventListener and the EventListener declares the `onEvent` method
* added EventHandler#dispatch and fixed EventDispatcher#dispatch
Added a TestCase for the event flow in the event system