Introduces Widget.LoadingDelegate, to allow a widget owner to act when a widget is added to or removed from the document without subclassing it. E.g. to attach to an event bus, or fire RPC requests. Review at http://gwt-code-reviews.appspot.com/866801 git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8770 8db76d5a-ed1c-0410-87a9-c151d255dfc7