Add support for action buttons on groups/sections

[pcdavid]

• Add org.eclipse.sirius.components.forms.Group.buttons : List<org.eclipse.sirius.components.forms.Button>
  • Update the GraphQL Schema, the GroupDescription, GroupComponent etc. to allow the description and rendering of these.
• Update FormEventFragments to fetch the new field Group { buttons }
• Update Group.tsx on the front to display the buttons in group's header
  • Actual UI to be defined.
  • Reuse ButtonPropertySection for the actual implementation, but this may be by copy/paste instead of trying to extract some shared code.
• Update view.ecore to allow the configuration of these buttons from the View DSL.
• Update the Form Description Editor to allow for adding/removing buttons on groups
  • The Form Description Editor does not support the definition of additional groups. It's basically for designing a single Group. So buttons will be dropped in a single new drop area at the top of the preview area.