- slots: * change name "initcmd" of "experimental slot features" to "defaultcmd" to avoid naming conflict the the initcmd executed at the initilization of a slot object (effects XOTcl as well) * make defaultcmd/valuecmd/valuechangedcmd working for nx (absence of trace method) * provide error message, when default/defaultcmd/valuecmd are used non-exclusively * make sure to init per-object slots after copy operations * make nx::Attribute.destroy more definsive * extend test cases