Add support for AVIF images

[Lonnie Lee Best]

Please add support of the AVIF image file format:
- https://en.wikipedia.org/wiki/AV1#AV1_Image_File_Format_(AVIF)

The latest Chrome and Firefox 87 support AVIF images, but Scribus won't accept them.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: scribus 1.5.5+dfsg-6build1
ProcVersionSignature: Ubuntu 5.4.0-66.74-generic 5.4.86
Uname: Linux 5.4.0-66-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Tue Mar 2 13:50:31 2021
InstallationDate: Installed on 2020-04-26 (310 days ago)
InstallationMedia: Kubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: scribus
UpgradeStatus: No upgrade log present (probably fresh install)

[Mattia Rizzolo (mapreri)]

I'm closing this bug as there isn't anything useful to be done on the Ubuntu side.

You can follow the related upstream bug instead: https://bugs.scribus.net/view.php?id=16377