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-zh-TW.json

@@ -456,10 +456,14 @@
     "full documentation": "完整說明文件",
     "items": "個項目",
     "seconds": "秒",
-    "theme-name-black": "黑色",
-    "theme-name-dark": "深色",
-    "theme-name-default": "預設",
-    "theme-name-light": "淺色",
+    "theme": {
+        "name": {
+            "black": "黑色",
+            "dark": "深色",
+            "default": "預設",
+            "light": "淺色"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} 想要共享資料夾 \"{{folder}}\"。",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} 想要共享資料夾 \"{{folderlabel}}\" ({{folder}})。",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} 可能會重新引入此裝置。"