ui/input: support single stepping through events

Change input.Events interface to return one event at a time until
the queue is empty.

Change text.Editor and gestures to match.

Re-add Editor.Submit while we're here; we don't want to enable
submit mode always.

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

ui/input/queue.go

@@ -19,8 +19,14 @@ type Queue struct {
 
 type handlerEvents map[Key][]Event
 
-func (q *Queue) For(k Key) []Event {
-	return q.handlers[k]
+func (q *Queue) Next(k Key) (Event, bool) {
+	events := q.handlers[k]
+	if len(events) == 0 {
+		return nil, false
+	}
+	e := events[0]
+	q.handlers[k] = events[1:]
+	return e, true
 }
 
 func (q *Queue) Frame(ops *ui.Ops) {