basic support for color-theme description and legend

src/apps/canvas/component/app.tsx

src/apps/canvas/component/structure-representation.tsx

src/apps/canvas/component/structure-view.tsx

src/apps/canvas/index.html

src/mol-model/structure/structure/carbohydrates/constants.ts

src/mol-util/color/color.ts

src/mol-util/color/scale.ts

src/mol-view/theme/color.ts

src/mol-view/theme/color/carbohydrate-symbol.ts

src/mol-view/theme/color/chain-id.ts

src/mol-view/theme/color/cross-link.ts

src/mol-view/theme/color/custom.ts

src/mol-view/theme/color/element-index.ts

src/mol-view/theme/color/element-symbol.ts

src/mol-view/theme/color/shape-group.ts

src/mol-view/theme/color/uniform.ts

src/mol-view/theme/color/unit-index.ts