Rework UX for adding digital signature
Currently when adding a signature the user is prompted to specify a rectangle, then chose a certificate. The problem here is that the user doesn't know what size is appropriate, so it takes some trial and error to get something good.
To address this change the flow a bit. First ask which certificate and other parameters should be used. With that information we create a signature annotation object and add that to the page. The user can then move and resize it like any annotation util the size and placement is right.
Once the user is happy with the placement they click on a Finish Signing button that then prompts for where to save the signed document and signs it.
This makes the warning about a too small signature rect redundant since the user sees that the signature won't fit. It was only an approximation anyway.
Needs https://gitlab.freedesktop.org/poppler/poppler/-/merge_requests/1560
Screen recording of the UX: