[2.0.0-beta2] UX: slots containers, wording length, source selectors

We need #title in each source. Otherwise the validation won't work. See https://www.drupal.org/project/ui_patterns/issues/3461277 📌 Sources don't support required - #title missing Active .
I think we should move this to beta2 and redesign the ui in general.

In discussion with Pierre we decided move to beta2 with a refactored ui

HTML only accordion:

<details>
  <summary>Prop1 <select><option>Source</option></select></summary>
  <div>Source form</div>
</details>
<details>
  <summary>Prop2 <select><option>Source</option></select></summary>
  <div>Source form</div>
</details>
<details>
  <summary>Prop3 <select><option>Source</option></select></summary>
  <div>Source form</div>
</details>