There is no event, but actually when you move an inventory item the unit is given an order, so you can catch it with the "Unit is issued an order ___...". However, it only has an integer order, not a string order.
So, to find out what the order is:
Trigger:
Events
Unit - A unit is issued an order with no target Unit - A unit is issued an order targeting an object
Yeah, it's something like 8522013. However, there are 6 different numbers, one each for the slot that the item is being moved to. So slot one might be 8522013, slot two would be 8522014, slot three 8522015, etc..