GWTII-231: remove CommonGwt inheritance in GeomajasClientApi.gwt.xml, ad...

add to server extensions gwt.xml

full build is succssful, see http://ci.geomajas.org/jenkins/job/Geomajas%20Project%20GWT2%20client%20issue%20231/

example of showcase with these changes: http://dev.geomajas.org/geomajas-client-gwt2-example-GWTII-231_moveInheritCommonGwtCommand/

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