engine: one ASCII fold, commitim through impre, no second modifier check

Four places lower-cased ASCII by hand; commitim repeated impre's three
lines to compute the text it commits; and transition() began by
rejecting modifier keys that imhandlekey's keymeaningful() had already
turned away, kept alive only by tests calling transition() directly.
The test helper now goes through the same gate.

tests/engine_test.c

@@ -10,7 +10,7 @@ keystroke(u32int ks, u32int mod, Str *com)
 {
 	int eaten;
 
-	eaten = transition(ks, mod, com);
+	eaten = keymeaningful(ks) && transition(ks, mod, com);
 	redraw();
 	return eaten;
 }