engine: the Japanese keyboard's own keys; keypad arrows move through candidates

半角/全角, ひらがな/カタカナ, 変換, and 無変換 committed the reading and did
nothing, like any function key.  They now stand for what a Japanese
keyboard means by them: toggling Japanese and English, switching kana
modes, converting (or turning Japanese on), and turning it off; the
Korean keys became a switch of the same shape.  The keypad's arrows
and page keys, with NumLock off, fold onto the plain ones as its Enter
and Tab already did.

README.md

@@ -17,7 +17,9 @@ Vietnamese Telex is also available as a compatibility mode.
 | `Ctrl+H` | One-shot Hanja search |
 
 A Korean keyboard's 한/영 key toggles Korean and English, and its 한자 key
-is `Ctrl+H`. Only a plain `Ctrl` chord is a strans key: with `Shift`, `Alt`,
+is `Ctrl+H`; a Japanese keyboard's 半角/全角 toggles Japanese and English,
+ひらがな/カタカナ switches between the two kana modes, 変換 turns Japanese on
+and then converts like `Space`, and 無変換 turns it off. Only a plain `Ctrl` chord is a strans key: with `Shift`, `Alt`,
 or `Super` held it commits what is pending and goes to the application, so
 `Ctrl+Shift+V` still pastes.