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-HK.json

@@ -498,10 +498,6 @@
     "full documentation": "完整文檔",
     "items": "條目",
     "seconds": "秒",
-    "theme-name-black": "theme-name-black",
-    "theme-name-dark": "theme-name-dark",
-    "theme-name-default": "theme-name-default",
-    "theme-name-light": "theme-name-light",
     "{%device%} wants to share folder \"{%folder%}\".": "{%device%} 想將 「{%folder%}」 文件夾共享給您。",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{%device%} 想要共享 \"{%folderlabel%}\" ({%folder%}) 文件夾給您。",
     "{%reintroducer%} might reintroduce this device.": "{%reintroducer%} 可能會重新引入此設備。"