Left separate from #8 because the fix & discussion will most likely go a little bit differently.

Issue:
In vanilla, shift-left-clicking a slot will transfer all items in it to a slot on the other side of the inventory. From what I can tell, this same functionality doesn't exist with WSlots.

Solutions:
The main issue is deciding where a slot should send items to (or where the "other side" is). A couple of ideas could be:

• having a default behavior (eg. opposite inventory ID, first slot) with overrideable behavior
• having priority or indexes for collections of slots as a whole
• using vanilla behavior (not sure if this would work with the custom slots, though, and it's my understanding it can bug out)

Requirements:
Ideally, shift-clicking would cover the normal functionality of:

• merging into existing stacks
• moving to the opposite side
• sending to "best fit" slot (eg. fuel -> furnace fuel slot, ore -> furnace burn slot)

it currently sends them to the first other inventory in the list. needs improving, of course.

Ahh, sorry about that. For some reason, it wasn't working previously (but is now working).