mol-plugin: custom plugin controls

src/apps/basic-wrapper/index.ts

src/apps/viewer/index.ts

src/examples/proteopedia-wrapper/index.ts

src/examples/proteopedia-wrapper/ui/controls.tsx

src/mol-plugin/context.ts

src/mol-plugin/layout.ts

src/mol-plugin/spec.ts

src/mol-plugin/ui/plugin.tsx