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-en.json

@@ -304,7 +304,6 @@
     "QR code": "QR code",
     "QUIC LAN": "QUIC LAN",
     "QUIC WAN": "QUIC WAN",
-    "QUIC connections are in most cases considered suboptimal": "QUIC connections are in most cases considered suboptimal",
     "Quick guide to supported patterns": "Quick guide to supported patterns",
     "Random": "Random",
     "Receive Encrypted": "Receive Encrypted",
@@ -538,15 +537,14 @@
     "modified": "modified",
     "permit": "permit",
     "seconds": "seconds",
-    "test": {
-        "translation": {
-            "dummy": "(This is just a test string for nested translation namespaces. This does not need to be translated.)"
+    "theme": {
+        "name": {
+            "black": "Black",
+            "dark": "Dark",
+            "default": "Default",
+            "light": "Light"
         }
     },
-    "theme-name-black": "Black",
-    "theme-name-dark": "Dark",
-    "theme-name-default": "Default",
-    "theme-name-light": "Light",
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} wants to share folder \"{{folder}}\".",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} wants to share folder \"{{folderlabel}}\" ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} might reintroduce this device."