mol-plugin: UI improvements (wip)

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

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

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

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

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

src/mol-plugin/ui/left-panel.tsx

src/mol-plugin/ui/plugin.tsx

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

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