Add rectangle elements to interaction widget

Test Plan:
Add an interaction widget and a rectangle
Change color of rectangle
Change position, width, and height
Add a movable point
Set position and height to be dependent on x_0 and y_0
Drag point around and see expected behavior

Reviewers: aria, alex

Reviewed By: alex

Subscribers: joel, michelle, alex

Differential Revision: http://phabricator.khanacademy.org/D14127