Add label support to interaction widget

Test Plan:
Add an interaction widget and label
Change the text of the label
Change the color of the label
Change the position of the label
Set the position of the label to (x_0, y_0 + 1) and add a movable point
Drag the point around and see the label follow

Reviewers: aria, alex

Reviewed By: alex

Subscribers: joel, michelle, alex

Differential Revision: http://phabricator.khanacademy.org/D14126