Add graphie.addReflectButton for Perseus Transformer widget

Summary:
Adds a function to place a reflection button, similar to the rotation
handle.

Test Plan:
Refactor perseus transformer widget to use this function, then test
 * moving the reflection line's points
 * dragging the reflection line itself
 * reflecting polygons

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5594