org.kde.kdenlive.appdata: Add developer_name

Fixes the following lint errors for Flathub:
```
{
    "errors": [
        "appstream-missing-developer-name"
    ]
}
```

See:
- https://github.com/flathub/org.kde.kdenlive/pull/329
- https://github.com/flathub-infra/flatpak-builder-lint/issues/274
- https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer
- https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name