Adds support for the new custom stamp icons APIs

This MR is used to implement my GSoC Project, add support for custom images stamp, it's based on Poppler's MR Improve Poppler Support For Custom Image Annotations.

This should enhance Okular's custom stamp support, by actually relying on the functionality to be added to Poppler (supporting adding custom stamps) and not go the way of rendering the stamps ourselves. This means that we can now add stamps from Okular (standard, or custom ones) and other viewers can render it correctly.

Here's what has been done so far:

• Modified the PopplerAnnotationProxy class to trigger the new functionality in Poppler
• Disabled Okular's rendering for stamps
• Store the stamp annotation appearance to not lose the appearance upon delete operation