menu: keyboard_layout: add ability for layout-specific settings

A layout might want to make some specific feature configurable, so
create an addToMainMenu-like system for allowing layouts to add their
own configuration sub-menu to the keyboard configuration menu.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

frontend/ui/widget/virtualkeyboard.lua

@@ -761,6 +761,10 @@ local VirtualKeyboard = FocusManager:new{
         ru = "ru_keyboard",
         tr = "tr_keyboard",
     },
+
+    lang_has_submenu = {
+        ja = true,
+    },
 }
 
 function VirtualKeyboard:init()