Ability to hide an inserter's held_stack

[Therax]

My mod interacts with inventories and belts via invisible inserters, which works well. viewtopic.php?f=93&t=54343
However, the item stacks in the hands of the invisible inserters are still visible, leading to the odd appearance of items stacks floating in mid-air. Could you add a flag to the inserter that suppresses that rendering?

[Rseding91]

Added for 0.16.