Support for deleting subform-layoutset

[framitdavid]

We need to support deleting layout sets in general, but in this task, we must specifically address deleting layout sets for subforms. How should we approach this?

Suggestion:
We notify the user that all subforms using the layout-set that is being deleted will also be removed. Then, we send a delete request to the backend, which will update all necessary locations. After that, the backend will provide us with information via web sockets about the files that have been changed, allowing us to invalidate queries on the frontend accordingly. (We do already have web-sockets and invalidaiton setup ready for use).

• When can delete a sub-form layout-sets (type: "sub-form")

[Jondyr]

Nice work! 🚀 It looks like the deletion of the layoutsets isn't fully implemented yet, as neither the applicationmetadata file nor the BPMN file are being updated when using the FileUpload and FileUploadWithTag components. So we might want to keep the issue linked to this PR open for now. 🤔

Originally posted by @mlqn in #13680 (review)

As @mlqn pointed out, deletion of layoutsets does not sync applicationmetadata or bpmn references, where fileupload might be relevant for subforms if they are allowed within the layout.

Update:
It seems fileupload components are restricted from being added to subform layouts (ref. #13475 ), so I believe this issue is complete without additional sync.

[nkylstad]

Tested OK:

• Added a subform layoutset
• Added a subform component
• Configured subform component to point at subform layoutset
• Deleted subform layoutset
• Confirmed "YES" in the dialog
  Result was that the subform layoutset, and the associated subform component in the main layoutset, were deleted as expected.

Also verified that the "Delete subform" button is only available when a subform layoutset is selected.

A few observations that probably go a little beyond this issue:

• The text in the dialog does not inform the user that the associated subForm component will be deleted.
• Deleting the subForm component does not delete the layoutset.

Since we are still working on the UX of how the user should experience managing subforms, I suggest we don't to anything about this for now, since we might be changing things anyways.
/cc: @Annikenkbrathen

The actual functionality of this issue (deleting the subform layout-set) seems to be working as expected 🥳