mol-plugin: Toast support

src/apps/basic-wrapper/controls.tsx

src/apps/basic-wrapper/index.html

src/apps/basic-wrapper/index.ts

src/mol-plugin/command.ts

src/mol-plugin/context.ts

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

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

src/mol-plugin/state/toast.ts

src/mol-plugin/ui/controls.tsx

src/mol-plugin/ui/plugin.tsx

src/mol-plugin/ui/toast.tsx