io/input: [API] introduce Source, the interface between a Router and widgets

This change gets rid of the event.Queue interface by replacing it with
input.Source values. Source provides the interface to Router necessary
to implement interface widgets.

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

widget/dnd_test.go

@@ -17,7 +17,7 @@ func TestDraggable(t *testing.T) {
 	var r input.Router
 	gtx := layout.Context{
 		Constraints: layout.Exact(image.Pt(100, 100)),
-		Source:      &r,
+		Source:      r.Source(),
 		Ops:         new(op.Ops),
 	}