Add image attachments support using GTK

[grynspan]

This issue tracks adding the equivalent of AttachableAsCGImage when using GTK. We presumably would want to add support for GdkPixbuf using gdk_pixbuf_save_to_bufferv().

Problems to solve:

• [ ] GTK is not guaranteed to be available, so needs to be weak-linked (?)
• [ ] GTK is not, AFAIK, Clang-modularized

GTK does not provide Swift language bindings at this time: https://www.gtk.org/docs/language-bindings/

[grynspan]

Filed https://gitlab.gnome.org/GNOME/gtk/-/issues/7782