flatpak: Re-indent manifest, update finish-args & deps
flatpak: Re-indent manifest
Match the indentation used by Flathub & flatpak-external-data-checker.
You can check that this diff is pure whitespace with:
git diff --ignore-space-change <commit>
flatpak: Add CUPS access to finish-args
flatpak: Update and add new dependencies
Matches the changes from Flathub.