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/material/slider.go

@@ -56,7 +56,7 @@ func (s SliderStyle) Layout(gtx layout.Context) layout.Dimensions {
 	trans.Pop()
 
 	color := s.Color
-	if gtx.Source == nil {
+	if !gtx.Enabled() {
 		color = f32color.Disabled(color)
 	}