Evaluate if better to provide a form element

Commit message.

Issue #3490142 by grimreaper, pdureau: Move styles manager form alter logic to a form element.

- do not save empty config entries
- Rework ui_styles_attributes source regarding last changes in UIP2
- Allow ui_styles_attributes source to set other attributes
- Fix ui_styles_ui_patterns config schema

@todo Create tests on the form element.

@pdureau I ask you some macro questions in MR.

Waiting feedbacks before converting stylesManager::alterForm tests to the Form element tests.

Very cool. OK with everything

Remove code that is now in https://www.drupal.org/project/ui_patterns/issues/3501400 ✨ DX: attribute prop source trait? Active

Automatically closed - issue fixed for 2 weeks with no activity.