Just a minimal code reduction, but big reduction in code
duplication. This could help if in the future, support for
different "internationalization" mods will be added, or in
case Minetest supports I18N/L10N internally in the engine.
Renamed the plain one to include "black", aliased to original name.
Better textures for the wooden table/chair and the black armchair.
Also renamed purple shutter to violet for consistency.
Also fixed a screwed up register_craftitem that was overriding a register_node
call for the same object (wooden table legs). Got rid of the old "mkII" wooden
table legs.