mol-plugin: 1st state saving prototype

src/mol-plugin/behavior/data.ts

src/mol-plugin/command/data.ts

src/mol-plugin/context.ts

src/mol-plugin/index.ts

src/mol-plugin/state.ts

src/mol-plugin/state/transforms/visuals.ts

src/mol-plugin/ui/controls.tsx

src/mol-plugin/ui/plugin.tsx

src/mol-plugin/ui/state-tree.tsx

src/mol-state/state.ts

src/mol-state/transformer.ts

src/mol-state/tree.ts

src/mol-state/util/immutable-tree.ts