API for MineClone 2 walls

This API allows you to add more walls (like the cobblestone wall) to MineClone 2.

`mcl_walls.register_wall(nodename, description, craft_material, tiles, invtex, groups, sounds)`

Adds a new wall type. This is optimized for stone-based walls, but other materials are theoretically possible, too.

The current implementation registers a couple of nodes for the different nodeboxes. All walls connect to solid nodes and all other wall nodes.

If craft_material is not nil it also adds a crafting recipe of the following form:

CCC
CCC

Yields 6 walls
C = craft_material (can be group)

nodename: Full itemstring of the new wall node (base node only). Must not have an underscore!
description: Item description of item (tooltip), visible to user
craft_material: Item to be used in the crafting recipe. If nil, no crafting recipe will be added
tiles: Wall textures table, same syntax as for minetest.register_node
inventory_image: Inventory image (optional, default is an ugly 3D image)
groups: Base group memberships (optional, default is {cracky=3})
sounds: Sound table (optional, by default default uses stone sounds)

The following groups will automatically be added to the nodes (where applicable), you do not need to add them to the groups table:

deco_block=1
not_in_creative_inventory=1 (except for the base node which the player can take)
wall=1

mcl_walls.register_wall("mymod:granitewall", "Granite Wall", {"mymod_granite.png"}, "mymod_granite_wall_inv.png")