widget,widget/material: export Enum.Value

The Value method both updated the enum value and returned it.

In order to access the current value withoutm, expose the Value
field of the enum and rename the method to Update. As a bonus we
can get rid of the SetValue method as well.

Updates gio#96

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

widget/material/radiobutton.go

@@ -32,7 +32,9 @@ func RadioButton(th *Theme, key, label string) RadioButtonStyle {
 	}
 }
 
+// Layout updates enum and displays the radio button.
 func (r RadioButtonStyle) Layout(gtx *layout.Context, enum *widget.Enum) {
-	r.layout(gtx, enum.Value(gtx) == r.Key)
+	enum.Update(gtx)
+	r.layout(gtx, enum.Value == r.Key)
 	enum.Layout(gtx, r.Key)
 }