Factor highlight animation into AnswerCellView
Add animation to split AnswerCellViews

Add the ability to toggle digits on AnswerCellViews.
The digits dont appear yet, but if there is more than one
an animation is triggered to split the cell into squares.
Draw and animate digits in answer cells

Make AnswerCellView draw its digits, and animate
their color and position
Hook number and clear buttons to grid

Hook up the functionality of the number and
clear buttons on the keyboard, so that they
toggle and remove digits on the selected grid
cell.
Animate digit translations independently of each other

Allow for more than one digit translation on an answer
cell at a time. This way if one is started while another
exists, the other can be reversed and the new animation
can continue smoothly.