io,app: route all unhandled key events to the topmost handler

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/key/key.go

@@ -30,6 +30,8 @@ type InputOp struct {
 	Hint InputHint
 	// Keys is the set of keys Tag can handle. That is, Tag will only
 	// receive an Event if its key and modifiers are accepted by Keys.Contains.
+	// As a special case, the topmost (first added) InputOp handler receives all
+	// unhandled events.
 	Keys Set
 }