org.kde.gwenview.appdata: Add developer_name & update caption

Fixes the following lint errors for Flathub:

{
    "errors": [
        "appstream-missing-developer-name"
    ],
    "warnings": [
        "appstream-screenshot-missing-caption"
    ]
}

See:

• https://github.com/flathub/org.kde.gwenview/pull/123
• 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