Add file link on image with CKE 5

The draft MR fixes the file link insert on images and generates whats seems like a valid data model and correct UI, however there is a remaining problem with the downcast: it generates two <a> tags.
I think one comes from drupalfileediting.js and the other from downcastBlockImageLink().

I'm not entirely sure this is the best approach but it seems to work for us.

There is a remaining problem: if the image already has a link on it, uploading a file does not change this link (in the data model).