mol-plugin: initial custom property support

src/mol-model-props/pdbe/structure-quality-report.ts

src/mol-plugin/behavior.ts

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

src/mol-plugin/context.ts

src/mol-plugin/index.ts

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

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

src/mol-plugin/util/custom-prop-registry.ts