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/input.go

@@ -7,7 +7,7 @@ package input
 // Events maps an event handler key to the events
 // available to the handler.
 type Events interface {
-	For(k Key) []Event
+	Next(k Key) (Event, bool)
 }
 
 // Key is the stable identifier for an event handler. For a handler h, the