Add inventory rmb_down drag drop

Like this: http://webmshare.com/o8QoL
Checks if right mouse button is down when entering inventory field and sets move_amount to 1 (drop 1 item to inv. field underneath, if it's nonempty with different item, it switches the stacks - that is caused by moving of the iteams being handled by existing code. Added code for pretty basic, so any specific behavior would require additional changes.

As far as I could tell, there isn't any fast way to determine if rmb is down, other than saving the state during pressed/released events. If there is more proper way, it should be used instead.