Interactive2: Pull constrain() out of modify()

Summary:
Moves the constrain() default for MovablePoints from modify() to add(),
since calling the constrain functions on every <Graphie> update was
annoying. (I'm not certain these semantics are perfect, but I'm
iterating.)

Also refactors some Interactive2 code around calling _applyConstraints,
which previously used a private method, meaning Interactive2 clients
couldn't replicate the functionality of MovablePoint.modify.constrain.

Test Plan: Used in numbnerline refactor

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D9135