Appdata: update the appdata per the latest specs
specs: https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html
This will allow having more info on https://flathub.org/apps/details/org.gnome.IconPreview :p
specs: https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html
This will allow having more info on https://flathub.org/apps/details/org.gnome.IconPreview :p