Add allowScratchpad option in graphie.addMouseLayer

Summary:
Small ke changes necessary to accomodate enabling the scratchpad for
all perseus exercises.

Test Plan: used in perseus when moving items above the scratchpad

Reviewers: brianmerlob, eater

Reviewed By: brianmerlob, eater

Subscribers: alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D7551