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?