Generate the appstream metadata of the gallery app

Summary:
I want to have a flatpak package with the gallery to test stuff and it
needs appstream info

Test Plan: The generated file validates successfully

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D7925