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