Icon as slot or prop

hello, i will take this one with you

hello, the change proposed is messing up something in the DerivableContextSourceBase,
but the solution is elsewhere.

drupal config required for slots and props is not the same.

for a prop, we have like:

prop_id:
   source_id: xxx
   source:
      ... (source config)

for slots, it is slightly different :

slot_id:
  sources:
    -
       source_id: xxx1
       source:
          ... (source config)
    - 
       source_id: xxx2
       source:
          ... (source config2)

if i am not mistaken, the correction you have posted in the MR is not required.
i am now investigating the rest to help you make it work for ui_icons :)

Good for me.