framework/element: Add element's SELECTED, MULTIPLE, REQUIRED attributes handling

... just like CHECKED and DISABLED

Consider adding `SingleSelection` component example based on `` .

Start by locating the framework/element implementation and the existing CHECKED and DISABLED attribute handling. Review how the SELECTED, MULTIPLE, and REQUIRED attributes should behave, then inspect the project’s existing component examples; the work is done when those attributes are handled consistently and a SingleSelection example based on is included.

Written by the indexing model from the issue text.