mol-plugin: refactored state saving

src/examples/proteopedia-wrapper/changelog.md

src/examples/proteopedia-wrapper/index.ts

src/mol-plugin/behavior/static/state.ts

src/mol-plugin/command.ts

src/mol-plugin/state.ts

src/mol-plugin/ui/plugin.tsx

src/mol-plugin/ui/state.tsx

src/mol-util/data-source.ts