Use wrapped MovablePoints

Summary:
Now that graphie has a fixedEllipse function, we use that in interactive2's MovablePoints.
As fixedEllipse returns both a visibleShape (the underlying SVG) and a wrapper (the <div> it's
wrapped in), we abstract away the distinction between these two objects using a new WrappedEllipse
class. Essentially, we will sometimes want to call methods on the wrapper (e.g., when transforming)
and, other times, will want to call methods on the SVG object (e.g., when changing its color). The
WrappedEllipse module gives us a common interface for interacting with the wrapped Movable at a
higher level and makes existing interactive2 code much more compatible with the wrapped MovablePoint.

Depends on D14158.

Test Plan:
- Run Perseus.
- Create an [interactive-graph](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20interactive-graph%202%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22interactive-graph%202%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B2%2C2%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-20%2C20%5D%2C%5B-20%2C20%5D%5D%2C%22graph%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A2%7D%2C%22correct%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A2%2C%22coords%22%3Anull%7D%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D) with multiple points.
- Verify that behavior is the same as before (i.e., points highlight on hover, grading works, etc.).
- Verify that the mouse targets and visible shapes are being added to separate <div>s.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D14159