Extract a Brick out of its Tower from the pie menu

[parthdagia05]

The extract wedge in #795: the Brick alone out of its Tower, the chain closing over the gap, so a wrapper can be removed without taking what it holds. detachBrickToNewTower carries the whole sub-tree out instead, which is what a drag wants.

Objectives:

• Sever the Brick alone, splicing its cavity contents and then its next chain into the gap.
• Handle each position the Brick can hold: in a chain, at the head of a cavity, and at a Tower's root, keeping the Tower's position so nothing below it moves.
• Leave the extracted Brick as a new Tower beside its old one, carrying its arguments.
• Let a folded Brick take its hidden contents with it, as it does on a drag in Handle dragging and dropping with folded Bricks #770, so only its next chain closes the gap.
• Route an Argument Brick to detachBrickToNewTower, and disable the wedge where extract would do nothing.
• Re-run the layout of both Towers and re-sync both Collision spaces.
• Cover it with tests: the splice in each position, the folded case, and the argument case.