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-ko-KR.json

@@ -538,10 +538,14 @@
     "modified": "수정됨",
     "permit": "허용",
     "seconds": "초",
-    "theme-name-black": "검은색",
-    "theme-name-dark": "어두운 색",
-    "theme-name-default": "기본 색",
-    "theme-name-light": "밝은 색",
+    "theme": {
+        "name": {
+            "black": "검은색",
+            "dark": "어두운 색",
+            "default": "기본 색",
+            "light": "밝은 색"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} 기기가 \"{{folder}}\" 폴더를 공유하길 원합니다.",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} 기기가 \"{{folderlabel}}\" ({{folder}}) 폴더를 공유하길 원합니다.",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} 기기에서 이 기기를 다시 소개할 수 있습니다."