org.kde.dolphin.appdata: Add developer_name

org.kde.dolphin.appdata: Add developer_name

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

See: https://github.com/flathub/org.kde.dolphin/pull/207
See: https://github.com/flathub-infra/flatpak-builder-lint/issues/274
See: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer
See: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name