Property "types" are handled by SlimeProperty instances. Whilst not allowing to create SlimeProperty objects, there is a list of all available properties. Properties and their values are stored in SlimePropertyMaps.
Example Usage:
// Create a new and empty property map
SlimePropertyMap properties = new SlimePropertyMap();
properties.setValue(SlimeProperties.DIFFICULTY, "normal");
properties.setValue(SlimeProperties.SPAWN_X, 123);
properties.setValue(SlimeProperties.SPAWN_Y, 112);
properties.setValue(SlimeProperties.SPAWN_Z, 170);
/* Add as many as you like */Properties can be modified after-the-fact aswell assuming it's loaded in the worlds.yml file.
SlimePlugin plugin = (SlimePlugin) Bukkit.getPluginManager().getPlugin("SwoftyWorldManager");
World world = // your world here
WorldsConfig config = plugin.getConfigManager().getWorldConfig();
WorldData worldData = config.getWorlds().get(world.getName());
// Other properties can also be changed, this example is just using the spawn property as an example
worldData.setSpawn(sender.getPlayer().getLocation().getBlockX() + ", " + sender.getPlayer().getLocation().getBlockY() + ", " + sender.getPlayer().getLocation().getBlockZ());
// Save config after the fact
config.save();