state & plugin: refactoring

src/mol-plugin/behavior/behavior.ts

src/mol-plugin/command/state.ts

src/mol-plugin/context.ts

src/mol-plugin/state.ts

src/mol-plugin/state/base.ts

src/mol-plugin/state/objects.ts

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

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

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

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

src/mol-state/object.ts

src/mol-state/state.ts

src/mol-state/tree/builder.ts

src/mol-state/tree/immutable.ts

src/perf-tests/state.ts