[UX] Provide an option to sort the keys/values of the options select form elements

This is basically to consider improving the option select elements that we've added in Backdrop core v1.12.0 (see #1005) to include the functionality of https://www.drupal.org/project/select_option_sort, which allows sorting the options in the select element by key (if the "Custom keys" checkbox is ticked) or value, and also ascending/descending.

Before:

After (suggested mockup):