Add image attachments support using GTK

[grynspan]

This issue tracks adding the equivalent of AttachableAsCGImage when using GTK.

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/