Implement backwards compatibility for fluid.prefs.enactors name change

fluid.prefs.enactors was changed to fluid.prefs.enactor after the 1.5 release. However, this can be considered as a backwards compatibility breaking api change. We should provide a grade or grades that essentially map fluid.prefs.enactors to fluid.prefs.enactor so that end users won't have to be affected by the change.

Note: fluid.prefs.enactors will still be deprecated and not available in the 2.0 release.