Group and sorting improvements

Problem/Motivation

Discussed today in UI Suite monthly meeting.

Ensure consistency across UI Suite ecosystem.

Proposed resolution

Now that groups had been introduced in UI Styles:
- Introduce a weight property to CSS variables
- Form: sort groups alphabetically
- Form: inside group sort CSS variables by weight then alphabetically
- Form: place CSS variables without groups in a "Other" group
- Form: if there is only the "Other" group, remove the group and place CSS variables directly in the CSS variables fieldset

Remaining tasks

- write tests dedicated to sorting
- review
- create change record
- synchronize merge and release with UI Styles and Ui Suite Bootstrap
- update project page