engine: the Korean keyboard's 한/영 and 한자 keys

A Korean keyboard sends Hangul and Hangul_Hanja for its two extra keys;
they did nothing but end the composition.  Now 한/영 toggles Korean and
English and 한자 opens the Hanja search, by standing in for Ctrl+S,
Ctrl+T, and Ctrl+H, as fcitx5-hangul binds them.

README.md

@@ -16,6 +16,9 @@ Vietnamese Telex is also available as a compatibility mode.
 | `Ctrl+E` | Emoji and symbol search |
 | `Ctrl+H` | One-shot Hanja search |
 
+A Korean keyboard's 한/영 key toggles Korean and English, and its 한자 key
+is `Ctrl+H`.
+
 A complete Japanese reading shows its Kanji candidates with none chosen.
 `Space` chooses the first and then steps on, wrapping (`Shift+Space` steps
 back); `Up`/`Down` and `PageUp`/`PageDown` move without wrapping. An