Enable scripted effects to load config translations from the kpackage

By default translations are loaded from /usr/share/locale. This works fine for builtin effects
since we install our translations there.

However, it doesn't work for third-party scripted effects. They only provide a kpackage and can't
install anything to /usr/share/.

This patch allows them to ship translations in contents/locale/$language/LC_MESSAGES/$domain.mo,
where $domain is the X-KWin-Config-TranslationDomain metadata value

This matches what Plasma does for applets since https://phabricator.kde.org/D5209

CCBUG: 317338

src/scripting/genericscriptedconfig.cpp

@@ -97,6 +97,11 @@ void GenericScriptedConfig::createUi()
         return;
     }
 
+    const QString localePath = packageRoot + QLatin1String("/contents/locale");
+    if (QFileInfo::exists(localePath)) {
+        KLocalizedString::addDomainLocaleDir(metaData.value("X-KWin-Config-TranslationDomain").toUtf8(), localePath);
+    }
+
     QFile xmlFile(kconfigXTFile);
     KConfigGroup cg = configGroup();
     KConfigLoader *configLoader = new KConfigLoader(cg, &xmlFile, this);