randomize layer and effect values

[bobnik]

This PR was inspired by #155, but it doesn't actually address the feature proposed there (though it could be easily added).

I've added a new "randomize value" button (die) for layers and effects. It makes sensible randomized choices for all layer and effect settings. It's pretty fun to play around with. The same option is available when creating layers and effects.

Random defaults and ranges can obviously be adjusted as needed as we play around with it.

[jeffeb3]

I didn't test all of these changes, but the randomize is pretty fun. Especially with shapes like the fractal spirograph where most options are interesting and there is a significant dynamic to changing the options.

I was going to suggest putting it on the shape options itself, instead of the new modal. But I worry that someone will accidentally clobber a bunch of work if they click it and don't have an undo button.

[bobnik]

I was going to suggest putting it on the shape options itself, instead of the new modal. But I worry that someone will accidentally clobber a bunch of work if they click it and don't have an undo button.

@jeffeb3 It is also in the shape options, though it's a little "dice" icon. You can keep clicking it to get different results each time.