mol-plugin: add custom props on demand, UI tweaks

src/mol-plugin/context.ts

src/mol-plugin/index.ts

src/mol-plugin/providers/custom-prop.ts

src/mol-plugin/providers/theme.ts

src/mol-plugin/skin/base/components/temp.scss

src/mol-plugin/state/actions.ts

src/mol-plugin/state/actions/structure.ts

src/mol-plugin/state/actions/volume.ts

src/mol-plugin/ui/controls.tsx

src/mol-plugin/ui/plugin.tsx

src/mol-state/action.ts

src/mol-state/action/manager.ts