Better reflection button and rotation handle options

Summary:
Added an option to set the degree width of the rotation handle, and
made the reflection button nicer to interact with, by adding better
click detection, and allowing dragging if constraints.fixed is not true.

Test Plan: Used in the new reflection line in perseus' transformer widget

Reviewers: alex, eater

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6221