add lua support for displaying formspecs for detached inventories

-added detached formspecs in order to be used for detached inventorys
-added new servermessage TOCLIENT_DETACHED_FORMSPEC to transfer detached formspecs to client
-added new servermessage TOCLIENT_SHOW_DETACHED_INVENTORY to trigger display of detached inventory on client
-added client support for showing gui on TOCLIENT_SHOW_DETACHED_INVENTORY message

This changes enable mods to show arbitrary formspecs to any client.