add API to PropertyQuestion enabling exporting quesion data to TestEnv but hiding it from CE

Questions which returns false for
boolean com.sun.interview.Question.isEnabled()
do not appear on the current path and so are skipped over by {@link Interview#prev} and {@link Interview#next}, etc.
And values of such questions are not exported to TestEnvironment!

But we need question with following properties:
- it is NOT appeared in CE(should be completely hidden from user)
- it is exported to TestEnvironment