wip: layout state

src/mol-plugin/command.ts

src/mol-plugin/component.ts

src/mol-plugin/context.ts

src/mol-plugin/layout.ts

src/mol-plugin/skin/base/components/controls.scss

src/mol-plugin/spec.ts

src/mol-plugin/ui/controls/common.tsx

src/mol-plugin/ui/viewport.tsx

src/mol-util/object.ts