allow different grid sizes (factors of 100 for now, but easily changed)

We also keep a partially transparent version of a grid for editmode. This
allows us to keep editmode visually distinct from runmode, even if snap-to-grid
isn't turned on.

If needed it's pretty easy to make the following changes:

* set the "big" cell size to something that isn't 100 (but I'm not sure how
  much control over the grid we actually want to give users-- e.g., setting
  the big cell to something for which the small cell isn't a factor
* allow snap-to-grid to be set independently for each canvas. (But again,
  what would be the use case for this?