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-ja.json

@@ -440,10 +440,14 @@
     "items": "項目",
     "modified": "更新",
     "seconds": "秒",
-    "theme-name-black": "ブラック",
-    "theme-name-dark": "ダーク",
-    "theme-name-default": "デフォルト",
-    "theme-name-light": "ライト",
+    "theme": {
+        "name": {
+            "black": "ブラック",
+            "dark": "ダーク",
+            "default": "デフォルト",
+            "light": "ライト"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} がフォルダー \"{{folder}}\" を共有するよう求めています。",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} がフォルダー「{{folderlabel}}」 ({{folder}}) を共有するよう求めています。"
 }