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-id.json

@@ -452,10 +452,14 @@
     "full documentation": "dokumentasi penuh",
     "items": "berkas",
     "seconds": "detik",
-    "theme-name-black": "Hitam",
-    "theme-name-dark": "Gelap",
-    "theme-name-default": "Bawaan",
-    "theme-name-light": "Terang",
+    "theme": {
+        "name": {
+            "black": "Hitam",
+            "dark": "Gelap",
+            "default": "Bawaan",
+            "light": "Terang"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} ingin berbagi folder \"{{folder}}\".",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} ingin berbagi folder \"{{folderlabel}}\" ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} mungkin memperkenalkan ulang perangkat ini."