Detached inventory callbacks and reworked node metadata callbacks

builtin/builtin.lua

builtin/deprecated.lua

builtin/detached_inventory.lua

builtin/item.lua

doc/lua_api.txt

games/minimal/mods/default/init.lua

games/minimal/mods/experimental/init.lua

src/guiFormSpecMenu.cpp

src/inventorymanager.cpp

src/scriptapi.cpp

src/scriptapi.h