engine: Backspace deletes the last kana shown; Tab converts like Space

Backspace undid a keystroke: な became ん, かんじ became かんj, きゃ
became ky.  Like every Japanese IME it now deletes the last kana as
shown — な goes, かんじ becomes かん, きゃ becomes き — while a romaji
letter that never became kana still goes one at a time.  Tab stepped
through candidates in a search and committed in Japanese; it steps
through them there too, with Space, and Shift+Tab steps back.

README.md

@@ -24,11 +24,12 @@ or `Super` held it commits what is pending and goes to the application, so
 `Ctrl+Shift+V` still pastes.
 
 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
-unmodified `1`-`9` commits that row of the current page. `Enter` and `Tab`
-commit the chosen candidate, or the reading when none is chosen, as `0`
+`Space` or `Tab` chooses the first and then steps on, wrapping (with
+`Shift`, back); `Up`/`Down` and `PageUp`/`PageDown` move without wrapping.
+An unmodified `1`-`9` commits that row of the current page. `Enter`
+commits the chosen candidate, or the reading when none is chosen, as `0`
 always does; so does typing on, or any key that ends the composition.
+`Backspace` deletes the last kana shown.
 Japanese consumes the confirming key; Korean and Vietnamese pass it on to
 the application. `Space` adds the reading in Katakana as the last
 candidate, so a word the dictionary lacks converts to Katakana with one