mol-plugin: better trajectory controls

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

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

src/mol-plugin/skin/base/icons.scss

src/mol-plugin/ui/controls.tsx

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

src/mol-plugin/ui/plugin.tsx

src/mol-state/state/selection.ts