inconsistant list increase/decrease behavior

[kyuwoo-choi]

initial list

after decrease list 2, decreased list item will be a part of parent list ( ul -> ol)

after increase list 3, increased list item have it's own parent list (ol -> ol)

I think the code below should be removed https://github.com/neilj/Squire/blob/306230d0df9b38047cd06204476ddc0582569cfd/source/Editor.js#L1509-L1513