[FEAT] Items in Belts

[Unknow0059]

Is your feature request related to a problem? Please describe.
I find myself having to imagine which items go where when I'm adding belts onto a blueprint, and I often lose track of what item goes where.

Describe the solution you'd like
It would be good if I could add items onto either side of belts. I could do that by selecting an item and then pressing Z with it in hand on either side of a belt.
That would then fill the entire side of the belt I placed it on. If there are inserters that take away from that belt, the entire side that the inserter puts an item on would be filled with the item it grabbed.

Describe alternatives you've considered
I tried using chests at the start of belt lines to represent different belts with items, but I can't open the chests in the editor, plus they wouldn't be able to represent lines with two different items.

[teoxoy]

Sounds like a good idea
Related to #22