Allow inventory items to be dragged and dropped in the world

A nice improvement to the inventory gui would be the ability to drag an item and drop it in the world. Currently it's only possible to drag items between different slots. It shouldn't be to hard to make the world render window a drop target, so that whenever an item is dragged to there and dropped it will be placed in the world. Once it's placed in the world it should be movable in the same way you can move any entity. When the user decides to place it it (by clicking the mouse button) the move operation should be sent to the server.