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/widget_test.go

@@ -22,7 +22,7 @@ func TestBool(t *testing.T) {
 		r   input.Router
 		b   widget.Bool
 	)
-	gtx := app.NewContext(&ops, app.FrameEvent{Source: &r})
+	gtx := app.NewContext(&ops, app.FrameEvent{Source: r.Source()})
 	layout := func() {
 		b.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 			semantic.CheckBox.Add(gtx.Ops)