Only resize the pic which was clicked on

Summary:
Requested on the widget improvement doc, as sad as I am to see this
change, I do think it is probably a good change.

Old:
{F16555}
New:
{F16556}

Test Plan:
* Open pic or dotplot
* Try dragging in an arc
* Should only change the one you clicked on

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D8447