(minor) Changed jQuery.NativeEventHandler to better work with jQuery.Widget

jQuery.Widget.prototype._trigger() works differently from jQuery.fn.trigger() which the
NativeEventHandler was originally optimized for. This is a refactoring of the tests so
the tests will be executed for both systems. At the same time this change set fixes
problems with using the NativeEventHandler with widgets. The main problem was that using
event.preventDefault() didn't have the desired effect. This is solved now. Also, the handling
didn't behave according to documentation when allowing custom results returned from native
handlers since this behavior is not supported by jQuery.Widget.prototype._trigger(). Another
problem was that the context the custom handlers were executed in were not sufficiently
documented in case of using the Widget variant.

Change-Id: I16689c120adadc387b98409a8a45b993013a876b