This adds a "B&W" button to the colors area in the cluster and mandala pages. Clicking it will reset the background, stroke and fill colors to their monochromatic defaults.
I'm calling the button "B&W" for now to minimize the amount of space the button takes up, since I'd like to add at least one more to that area. At some point we might want to use a UI that scales a bit better, like a dropdown button.
This replaces the "randomize colors" and "randomize" buttons on the creature and mandala pages with a single randomizer widget that allows the user to choose whether they want to randomize the colors, symbols, or both.
By default, both colors and symbols are randomized.
Note that this is really only a temporary solution, though: ideally we'll want to provide a more powerful UI that allows users to mark _any_ parameter for randomization, e.g. via "pinning" values or somesuch.
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.
Atul Varma