Add "large canvas view" to Desktop CMU graphics

[austin-schick]

The web interface includes a "large canvas view" button that increases the size of the canvas and automatically scales the image. That way students can see their drawing in a larger canvas, or play their game in a larger canvas, without having to draw all of their shapes at new sizes.

We should include an equivalent feature for Desktop CMU Graphics, maybe as an app method (something like app.resizeAndScale(600, 600))