gui: Use nested namespace for theme name translation keys (#9220)

Following up on #9192, this makes use of the new mechanism for the theme
names.

The dummy string added for testing is removed again here. All
translations are updated to the new nested syntax, except Chinese
(zh-HK) where the string weren't actually translated.

gui/default/assets/lang/lang-ru.json

@@ -497,10 +497,14 @@
     "items": "элементы",
     "modified": "изменено",
     "seconds": "сек.",
-    "theme-name-black": "Чёрная",
-    "theme-name-dark": "Тёмная",
-    "theme-name-default": "По умолчанию",
-    "theme-name-light": "Светлая",
+    "theme": {
+        "name": {
+            "black": "Чёрная",
+            "dark": "Тёмная",
+            "default": "По умолчанию",
+            "light": "Светлая"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} хочет поделиться папкой «{{folder}}».",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} хочет поделиться папкой «{{folderlabel}}» ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} может повторно рекомендовать это устройство."