defense_walls adds degradation and repairing for walls

Author of source code:

NetherEran (GPLv3)

Authors of media:
alegemaate (CC BY 3.0)
	defense_walls_mortar_use.ogg
inspectorj (CC0 1.0)
	defense_walls_mortar_application.ogg
	
both sounds have been edited and converted to ogg by NetherEran

NetherEran(CC BY-SA 3.0)
	degradable_walls_mortar_and_pestle.obj
	degradable_walls_mortar.png
	degradable_walls_mortar_and_pestle_inv.png
	degradable_walls_mortar_and_pestle_m.png
	degradable_walls_mortar_and_pestle_p.png

paramat (CC BY-SA 3.0)
	degradable_walls_wall.png

See license.txt for better license information.
