nx.tcl

- Use mixinregObjType as well in NsfRelationCmd(), so this is the only place, where mixin and guards are processed. - Since the type converter converts Tcl-Objs, we have less context information (e.g. we have no base class, on which we can decide to call e.g. __unknown on on of the objects systems). - because of the point above, i removed ::xotcl::Class->__unknown and ::nx::Class->__unknown in favor of a global proc ::nsf::unknown, for which unknown handlers can be registered - GetClassFromObj() receives as last argument "withUnknown" instead of baseClass to indicate, when unknown should be tried. - new function NsfCallUnkownHandler()