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>

layout/list_test.go

@@ -70,7 +70,7 @@ func TestListPosition(t *testing.T) {
 		Constraints: Constraints{
 			Max: image.Pt(20, 10),
 		},
-		Source: r,
+		Source: r.Source(),
 	}
 	el := func(gtx Context, idx int) Dimensions {
 		return Dimensions{Size: image.Pt(10, 10)}