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.

strans.c

@@ -356,13 +356,20 @@ commitim(Im *p, Str *com)
 	sclear(&p->raw);
 }
 
+/* Deletes the last kana as shown, not the last key: kya loses ゃ. */
 static void
 backjp(Im *p)
 {
-	if(p->raw.n != 0)
-		spopr(&p->raw);
-	else
+	Str v;
+
+	if(p->raw.n == 0)
 		spopr(&p->pre);
+	else if(mapget(p->l->map, &p->raw, &v)){
+		spopr(&v);
+		sappend(&p->pre, &v);
+		sclear(&p->raw);
+	}else
+		spopr(&p->raw);
 }
 
 /*
@@ -800,9 +807,10 @@ transition(u32int ks, u32int mod, Str *com)
 		reset();
 		return 1;
 	}
-	if(ks == ' ' && isjp(&im) && haspre(&im) && !(mod & ~Mshift)){
-		/* Space converts: it steps through the candidates, Katakana
-		 * last; a reading with none is committed. */
+	if((ks == ' ' || ks == Ktab) && isjp(&im) && haspre(&im) &&
+	   !(mod & ~Mshift)){
+		/* Space and Tab convert: they step through the candidates,
+		 * Katakana last; a reading with none is committed. */
 		katakouho();
 		if(im.nkouho != 0)
 			cyclekouho(mod & Mshift ? -1 : 1);