[UX] Allow keyboard layout selection (#5318)

Fixes <https://github.com/koreader/koreader/issues/3998>.

This isn't quite what I want but I've been letting this slip for
too long due to fancier keyboard interaction concepts.

frontend/ui/elements/reader_menu_order.lua

@@ -57,6 +57,7 @@ local order = {
         "status_bar",
     },
     device = {
+        "keyboard_layout",
         "time",
         "battery",
         "autosuspend",