Add PluginContext.initContainer/canvas3dInitialized and their usage

CHANGELOG.md

src/apps/docking-viewer/index.ts

src/examples/alpha-orbitals/controls.tsx

src/examples/alpha-orbitals/index.ts

src/mol-plugin-ui/index.ts

src/mol-plugin-ui/react18.ts

src/mol-plugin/context.ts