added StructureElement.Stats and improved loci-labels

src/mol-model/structure/structure/element.ts

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

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

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

src/mol-plugin/util/structure-element-selection.ts

src/mol-theme/label.ts

src/tests/browser/render-shape.ts