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-uk.json

@@ -507,10 +507,14 @@
     "items": "елементи",
     "modified": "змінено",
     "seconds": "секунд",
-    "theme-name-black": "Чорна",
-    "theme-name-dark": "Темна",
-    "theme-name-default": "Стандартна",
-    "theme-name-light": "Світла",
+    "theme": {
+        "name": {
+            "black": "Чорна",
+            "dark": "Темна",
+            "default": "Стандартна",
+            "light": "Світла"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} хоче поділитися папкою \"{{folder}}\".",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} хоче поділитися папкою \"{{folderLabel}}\" ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} може повторно порекомендувати цей пристрій."