Remove gwt-dev from the dependency list; clean up enforcer exceptions

Replace com.google.gwt.core.client.GWT with com.google.gwt.core.shared.GWT.
Introduce gwtmockito for tests which need to create ClickEvents:
   https://groups.google.com/d/msg/google-web-toolkit/H0Blv3C_fns/35PqYGlGvQ8J