engine: a mode switch shows the mode it switched to

Ctrl+S, Ctrl+N and the rest changed the mode with nothing to see; the
next key was the only way to tell.  The popup now shows A, 한, あ, ア, or
ă until a key is typed, beside the ☺ and 漢 a search already shows.

README.md

@@ -17,6 +17,9 @@ compatibility mode.
 | `Ctrl+E` | Emoji and symbol search |
 | `Ctrl+H` | One-shot Hanja search |
 
+Switching mode shows the mode switched to — `A`, `한`, `あ`, `ア`, `ă` — in
+the popup until the next key.
+
 A Korean keyboard's 한/영 key toggles Korean and English, and its 한자 key
 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