PDF Annotation - Add Signatures not as Annotations but fixed, non-removable changes

Thanks to #787 it is now possible to add images to a PDF. However, you cannot use this feature to collect signatures among several users. Why?

Whenever a user adds/uploads his own signature as an image and saves, every other user can still open the PDF in a PDF viewer and copy/move that added image as he likes. This is not helpful, as others can easily copy the signature and place them to any other document.

Proposed Solution
It is also possible to add images to PDFs which then are not stored as annotations but are added to the PDF - later editing of that object is no longer possible. A good example is macOS "Preview" app which allos to add your own signature. After saving, no one can remove it or copy it.

This would be a very powerful feature to collect simple signatures by multiple parties in a single PDF. A lot of business processes do not need a "qualified electronic signature", but PDF signatures would be fine. With this we could reduce big efforts in collecting signatures from several parties.

Start by reviewing the existing image-annotation feature from #787 and how macOS Preview makes saved signatures non-editable. Define how an uploaded signature should be incorporated into the PDF rather than retained as a removable annotation, and verify that other users cannot move, copy, or remove it after saving.