Handler clean up

Make sure that a few handlers are cleaned up
UiBinder implementation

A lot of events and handlers changed - getAssociatedType and
implementation of EventHandler (the Google one).
Some Google GWT core files altered to use third party parsers.
ImageView UiBinder changes, add HasWidgetsParser

Make it possible to load an ImageResource in UiBinder on an ImageView.
And also add HasWidgetsParser, used to parse the UiBinder xml files.