widget/material: allow configuring default typeface on theme

This commit introduces the material.Theme.Face field, which will automatically
populate the Font.Typeface in every text widget created using a constructor function
in package material.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/material/radiobutton.go

@@ -17,7 +17,7 @@ type RadioButtonStyle struct {
 // RadioButton returns a RadioButton with a label. The key specifies
 // the value for the Enum.
 func RadioButton(th *Theme, group *widget.Enum, key, label string) RadioButtonStyle {
-	return RadioButtonStyle{
+	r := RadioButtonStyle{
 		Group: group,
 		checkable: checkable{
 			Label: label,
@@ -32,6 +32,8 @@ func RadioButton(th *Theme, group *widget.Enum, key, label string) RadioButtonSt
 		},
 		Key: key,
 	}
+	r.checkable.Font.Typeface = th.Face
+	return r
 }
 
 // Layout updates enum and displays the radio button.