support Task cancellation in plugin UI

src/mol-plugin/context.ts

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

src/mol-plugin/ui/plugin.tsx

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

src/mol-plugin/ui/task.tsx

src/mol-task/execution/observable.ts

src/mol-task/task.ts

src/mol-util/data-source.ts