wip, custom structure props

src/mol-model-props/common/custom-element-property.ts

src/mol-model-props/common/custom-property-registry.ts

src/mol-model-props/computed/secondary-structure.ts

src/mol-plugin/behavior/dynamic/custom-props.ts

...vior/dynamic/custom-props/computed/secondary-structure.ts

...ior/dynamic/custom-props/pdbe/structure-quality-report.ts

...n/behavior/dynamic/custom-props/rcsb/assembly-symmetry.ts

src/mol-plugin/context.ts

src/mol-plugin/index.ts

src/mol-plugin/state/actions/structure.ts

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