added popup menu actions for hypothesis (right click on button)
to customize it, change the following fuction in ProofSubgoal.java

        static String [] getContexts()
        {
            return new String [] {
                        "invertsn (*s*). ",
                        "clear (*s*). ",
                        "duplicate (*s*). ",
                                    };
        }
Merge branch 'master' of github.com:aa755/nbcoq.git