Focus/Selection UI tweaks

src/mol-plugin-state/manager/structure/selection.ts

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

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

src/mol-plugin-ui/structure/focus.tsx

src/mol-plugin-ui/structure/selection.tsx

src/mol-plugin-ui/viewport.tsx

src/mol-util/binding.ts