ui/input: change Queue to return Events one at a time

By returning all events, widgets that might return early from its
event loop might throw away subsequent events. Instead of requiring
those widgets to store the event list, convert input.Queue to step
through the available events one at a time.

Functional revert of 1735d5ced8.

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

ui/input/input.go

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