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/switch.go

@@ -55,7 +55,7 @@ func (s SwitchStyle) Layout(gtx layout.Context) layout.Dimensions {
 	if s.Switch.Value {
 		col = s.Color.Enabled
 	}
-	if gtx.Source == nil {
+	if !gtx.Enabled() {
 		col = f32color.Disabled(col)
 	}
 	trackColor := s.Color.Track