widget/material: move widget state object from Layout methods to constructors

Instead of, say,

	var th *material.Theme
	var btn *widget.Clickable

	material.Button(th, "Click me").Layout(gtx, btn)

move the widget state objects to the constructor:

	material.Button(th, btn, "Click me").Layout(gtx)

The advatage is that several widgets can now be used without
wrapping them in function literals. For example,

	layout.Inset{}.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
		material.Button(th, "Click me").Layout(gtx, btn)
	})

collapses to just

	layout.Inset{}.Layout(gtx, material.Button(th, btn, "Click me").Layout)

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

widget/material/radiobutton.go

@@ -10,13 +10,15 @@ import (
 
 type RadioButtonStyle struct {
 	checkable
-	Key string
+	Key   string
+	Group *widget.Enum
 }
 
 // RadioButton returns a RadioButton with a label. The key specifies
 // the value for the Enum.
-func RadioButton(th *Theme, key, label string) RadioButtonStyle {
+func RadioButton(th *Theme, group *widget.Enum, key, label string) RadioButtonStyle {
 	return RadioButtonStyle{
+		Group: group,
 		checkable: checkable{
 			Label: label,
 
@@ -33,9 +35,9 @@ 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) layout.Dimensions {
-	enum.Update(gtx)
-	dims := r.layout(gtx, enum.Value == r.Key)
-	enum.Layout(gtx, r.Key)
+func (r RadioButtonStyle) Layout(gtx layout.Context) layout.Dimensions {
+	r.Group.Update(gtx)
+	dims := r.layout(gtx, r.Group.Value == r.Key)
+	r.Group.Layout(gtx, r.Key)
 	return dims
 }