Lifecycle method called when unbinding the object.
Important : Make sure you call your parent class
#onUnbind().
Also, do not call directly, call
#unbind() instead.
Any handler registration recorded with
#registerHandler(HandlerRegistration)will have
already been removed at this point. You should override this method to
take down any other initialisation that took place in
#onBind().
This method will never be invoked more then once, or if it is, the second
time will necessarily be preceded by an invocation of
#onBind().