feat(ui): Show language banner image as fallback when country image is unavailable

[Copilot]

When a locale has only a language component (e.g. de, fr, zh) and no country code, the properties editor always showed a blank placeholder image instead of any meaningful banner.

Change

In BundleEntryComposite.loadCountryIcon, added an else if fallback that attempts to resolve a banner using the language code when no country code is present:

if (countryCode != null && countryCode.length() > 0) {
    image = UIUtils.getImage("countries/" + countryCode.toLowerCase() + ".gif");
} else if (countryLocale != null
        && countryLocale.getLanguage() != null
        && countryLocale.getLanguage().length() > 0) {
    // show language image when country image is not available
    String languageCode = countryLocale.getLanguage();
    image = UIUtils.getImage("countries/" + languageCode.toLowerCase() + ".gif");
}
// existing blank.gif fallback remains unchanged

Port of essiembre/eclipse-rbe#62.