GJP-50: enable adding map controller as a listener

needs vote

example @ http://dev.geomajas.org/geomajas-project-javascript-gwt2-example-GJP-50_ControllerListener/examples/controller_as_listener.html 
(example part of general section in showcase http://dev.geomajas.org/geomajas-project-javascript-gwt2-example-GJP-50_ControllerListener/)