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-bg.json

@@ -538,10 +538,14 @@
     "modified": "променено",
     "permit": "разрешаване",
     "seconds": "секунди",
-    "theme-name-black": "Черна",
-    "theme-name-dark": "Тъмна",
-    "theme-name-default": "По подразбиране",
-    "theme-name-light": "Светла",
+    "theme": {
+        "name": {
+            "black": "Черна",
+            "dark": "Тъмна",
+            "default": "По подразбиране",
+            "light": "Светла"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} споделя папката „{{folder}}“.",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} споделя папката „{{folderlabel}}“ ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "Поръчителят {{reintroducer}} може отново да предложи това устройство."