Turn reflectButton into two triangles instead of a circle

Summary:
Replaces the circle point created by addReflectButton with a pair of
triangles.

Test Plan:
 * Use it in the transformer widget from D5596
 * Try adding a reflect tool and dragging the line around
 * Try reflecting points
 * Try removing and adding the reflect tool and check that the old
   button is not still clickable

Reviewers: alex

Reviewed By: alex

CC: alpert, khirasaki

Differential Revision: http://phabricator.khanacademy.org/D5601