Hilight squares under item when hovering an item in your hand over the inventory

[wheybags]

see #358 for discussion

Imo we should also abstract the rendering of the inventory into a function that takes a position in the gui and a BasicInventory and handles the whole thing while we're at it. That way we could use the same function for the main inventory and the belt (and maybe even the equip slots, but they're as bit funny so maybe not too).