interceptor-slot.test

nx.tcl: - add "set" as a method name for relation slots - implemented relation slot "mixin" and "object-mixin" via "slotassign" to disallow "/obj/ mixin /value/" and "/obj/ object mixin /value/" to use instead "/obj/ mixin set /value/" and "/obj/ object mixin set /value/" while keeping "configure" and "cget" working. This has the advantage that "/obj/ mixin set" does not try to replace the mixin chain by "set" - adapted regression test - TODO: check, if we need the explicit "slotassign"? isn't the presence of the slotObj sufficient? maybe "-forwardToSlot" in relationSlots? - TODO: demangle "slotassign" in "ObjectParameterSlot protected method getParameterOptions" and check interactions - TODO: to the same as -mixin and -object-mixin to -filter and -object-filter - TODO: clean up relation slot mixin variants - TODO: do we really like the fact that we have to write now "B mixin set M2" instead of "B mixin M2"? - TODO: should we disallow "B mixin" and enforce instead of "B mixin get" ? - TODO: we could as well allow "B mixin clear" instead of "B mixin set {}" - TODO: allow "set" for variable slots as well. Do we need "assign"?