mol-plugin: loci label support

src/mol-plugin/behavior/dynamic/representation.ts

src/mol-plugin/context.ts

src/mol-plugin/index.ts

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

src/mol-plugin/ui/controls.tsx

src/mol-plugin/ui/plugin.tsx

src/mol-plugin/util/loci-label-manager.ts