added Structure.root

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

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

src/mol-plugin/state/transforms/representation.ts

src/mol-plugin/ui/sequence/hetero.ts

src/mol-plugin/ui/sequence/polymer.ts

src/mol-plugin/util/interactivity.ts

src/mol-plugin/util/structure-overpaint-helper.ts

src/mol-repr/structure/complex-representation.ts

src/mol-repr/structure/complex-visual.ts

src/mol-repr/structure/units-representation.ts

src/mol-repr/structure/units-visual.ts

src/mol-repr/structure/visual/util/polymer.ts