Collapse transformations together if possible

Summary:
If the last two transformations in the transformer widget's list of
transformations are of the same type and can be combined into one, we
now combine them. This allows two nice results:
 * Users can see transformations as they apply them. Instead of waiting
   for a tranformation to finish before applying it, we can apply it at
   each step, resulting in a UI that shows the mathematical definition
   of the transformation as it is created.
 * Users can "adjust" their previous transformation, instead of making a
   long list of rotations about the same point.

Test Plan:
Visit test.html and apply each of the four transformations in
exploratory mode, making sure that it displays the correct text and
that it updates dynamically while applying it. Also verify for each
that a repeated transformation about the same point(s) modifies the
original transformation rather than creating a new item in the list.

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5501