widget: add semantic descriptions

Some semantic information is automatically extracted, but some must be
provided by UI components. This change enriches the generic and material
widgets with such information.

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

widget/material/checkbox.go

@@ -3,6 +3,7 @@
 package material
 
 import (
+	"gioui.org/io/semantic"
 	"gioui.org/layout"
 	"gioui.org/unit"
 	"gioui.org/widget"
@@ -32,6 +33,7 @@ func CheckBox(th *Theme, checkBox *widget.Bool, label string) CheckBoxStyle {
 // Layout updates the checkBox and displays it.
 func (c CheckBoxStyle) Layout(gtx layout.Context) layout.Dimensions {
 	return c.CheckBox.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
+		semantic.CheckBox.Add(gtx.Ops)
 		return c.layout(gtx, c.CheckBox.Value, c.CheckBox.Hovered())
 	})
 }