Custom boxy nodes (stairs, slabs) and collision changes

games/minimal/mods/default/init.lua

games/minimal/mods/stairs/depends.txt

games/minimal/mods/stairs/init.lua

src/camera.cpp

src/collision.cpp

src/collision.h

src/content_cao.cpp

src/content_mapblock.cpp

src/content_sao.cpp

src/game.cpp

src/localplayer.cpp

src/mapnode.cpp

src/mapnode.h

src/nodedef.cpp

src/nodedef.h

src/player.cpp

src/scriptapi.cpp

src/test.cpp

src/tile.h