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-si.json

@@ -502,10 +502,14 @@
     "modified": "සංශෝධිතයි",
     "permit": "අවසරය",
     "seconds": "තත්පර",
-    "theme-name-black": "කළු",
-    "theme-name-dark": "අඳුරු",
-    "theme-name-default": "පෙරනිමි",
-    "theme-name-light": "දීප්ත",
+    "theme": {
+        "name": {
+            "black": "කළු",
+            "dark": "අඳුරු",
+            "default": "පෙරනිමි",
+            "light": "දීප්ත"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} ට \"{{folder}}\" ෆෝල්ඩරය බෙදා ගැනීමට අවශ්‍යයි.",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} ට \"{{folderlabel}}\" ({{folder}}) ෆෝල්ඩරය බෙදා ගැනීමට අවශ්‍යයි.",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} මෙම උපාංගය නැවත හඳුන්වා දිය හැක."