Add bounding to points and lines

Test Plan:
Add an interaction widget, a movable point, and a movable line segment
Drag the point around to see that I can't drag it off the canvas
Change the constraints and drag the point around to see that it complies
Drag the line around to see that I can't drag it off the canvas
Change the constraints and drag the line around to see that it complies

Reviewers: aria

Reviewed By: aria

Subscribers: joel, michelle, alex

Differential Revision: http://phabricator.khanacademy.org/D14124