Add get_wielded_itemstring, get_wielded_item, damage_wielded_item and make getacceleration visible in API

Create the necessary API for /giveme and /give and implement those commands; also sort out the scripts a bit

Rename menu background to menubg.png, move unknown_block.png and unknown_object.png to client, make generating a placeholder image for the filename "" not cause an error

Make map generator as much threaded as possible (not much benefit with current generator because of small generator chunk size (a single MapBlock))