nx.tcl: pluralism reform part 3
- introduced simple plural form "mixins" and "filters" for introspection
- moved differentiated interface into slot methods.
  the slot methods "get" stay symmetrically to "set",
  but additional methods "classes" or "methods" are used
  like "guard" to obtain partial results of the
  mixin and filter specs
- changed info methods
    /cls/ info mixin classes         -> /cls/ info mixins
    /cls/ info filter methods        -> /cls/ info filters
    /obj/ info object mixin classes  -> /obj/ info object mixins
    /obj/ info object filter methods -> /obj/ info object filters
- dropped
    /cls/ info mixin guard
    /cls/ info filter guard
    /obj/ info object mixin guard
    /obj/ info object filter guard
- added
    /cls/ mixin classes
    /cls/ filter methods
    /obj/ object filter methods
    /obj/ object mixin classes