HEIF/HEIC support

Adds support for using GDK pixbuf support for HEIF/HEIC format file. Write support isn't yet present, so thumbnails are stored as JPEG. Also certain GDK pixbuf formats may return as "heif/avif", so we have special code to handle such a case (since we should have already gone through the is_heif_file test, this should be fine).

Fixes #203 (closed)