Minimize number of 'constant' actors
Current Kepler has 2 types of Constant actors (Constant and SingleFireConstant) and 2 different 'StringConstant' actors (PermanentStringConstant and StringConstant). In both cases, the primary difference is the number of times the actor 'fires'.
Ptolemy has now changed the behavior of its Constant actor to have a 'firingCountLimit' parameter. There is thus no need for the two types of 'constant' actors in Kepler (which have tended to confuse several users).
Note also that Ptolemy extends 'Const' to create 'StringConstant'. Kepler, however, does not! And Kepler's StringConstant has added a File parameter to StringConstant so that file name strings can be easily created.
We should change the StringConstant in Kepler to extend Constant AND add the File parameter.