Bunch of upgrades

- Events:
Changed CloseEvent (and related) to GWT's implementation. Changed HasCloseHandler to extend from GWT's impl and added support to the ClosedEvent.
Changed the Closed, Hide, Hidden, Show and Shown events to support auto triggered attributes.

- Changed the Modal to support firing the auto triggered attribute in the hide/hidden and show/shown events.

- Changed the AlertBase to fire GWT's CloseEvent. (Later, I will implement the support for firing auto triggered close event).

- Added a little logic to prevent DropdownBase to break when some impl do not implement a trigger. (Later, I intend to pull an own impl of Dropdown that does not have a trigger)