Add a sortable checkboxes widget (tableselect) as advanced OptionsButtonsWidget alternative

Postponed on #3487726: FormElement tableselect is incompatible with #tabledrag
Contrib implementation in: #3487683: Add "Check all" functionality (switch to "tableselect") (also postponed on the core issue)

Problem/Motivation

OptionsButtonsWidget provides the

Check boxes/radio buttons

field widget.

Sadly, in contrast to EntityReferenceAutocompleteWidget those checkboxes / radios are not sortable, while they are still stored with delta value in the database already.
Switching to EntityReferenceAutocompleteWidget is not an option for UX reasons often, if you want to show all available options to the user.

So currently you don't have best of both worlds combined in the UI for the user as widget:

• Checkboxes / Radios
• Sorting
• (Bonus: Check all functionality - also see #3459246: Add a '#check_all' option for checkboxes element)

As Drupal already provides the wonderful tableselect FAPI element and already stores the delta in database, it should be relatively easy and benefitial to provide a "Sortable check boxes/radio buttons" widget in core.

Steps to reproduce

Proposed resolution

Provide a "Sortable check boxes/radio buttons" widget in core to allow not only (bulk) checking checkboxes option but also allow sorting the values (delta) if needed.

Remaining tasks

1. Discuss
2. Implement
3. Test
4. Release

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet