This fixes#67 by making the background color selection/randomization code more DRY via the addition of a new `SvgCompositionContext` and a `CompositionContextWidget`.
It also documents the `SvgSymbolContext` type, and moves the "randomize colors" button closer to the actual colors.
The key phrase here is **less terrible**, as opposed to **good**.
Right now if the mandala gets too big for the page, it will just overflow and the remainder won't be visible (not even via scrolling). This is intentional, though, as per @ninapaley's suggestion:
> I'm hoping you can change the way the image keeps resizing its canvas, and keeps justifying left.
> Maybe have an oversize canvas with the ring pinned to the center, and all the adjustment sliders
> on the right? Then they won't disappear if the canvas is too big. Also! I have an idea for an
> animated cycle that would require the symbols to eventually outgrow the canvas area and disappear.
I'm not super happy with this CSS, it doesn't feel terribly maintainable. Ah well, maybe we can improve it later.
Atul Varma