feat(ui): style app menus and extract glossary dialog

- Add AppMenu and AppMenuItem so menu labels, mnemonics and shortcuts render consistently under org.kde.union
- Move the translation glossary into GlossaryDialog with add, remove and clear-all actions
- Regroup the translation preset and instructions settings layout

src/translate/translationservice.cpp

@@ -117,7 +117,7 @@ QVariantList TranslationService::translationPresets() const
 {
     QVariantList result;
     for (const auto &preset : TranslationPresets::all()) {
-        result.append(QVariantMap{{u"id"_s, preset.id}, {u"name"_s, preset.name}, {u"guidance"_s, preset.guidance}});
+        result.append(QVariantMap{{u"id"_s, preset.id}, {u"name"_s, preset.name}, {u"description"_s, preset.description}, {u"guidance"_s, preset.guidance}});
     }
     return result;
 }