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/checkbox.go

@@ -14,7 +14,7 @@ type CheckBoxStyle struct {
 }
 
 func CheckBox(th *Theme, checkBox *widget.Bool, label string) CheckBoxStyle {
-	return CheckBoxStyle{
+	c := CheckBoxStyle{
 		CheckBox: checkBox,
 		checkable: checkable{
 			Label:              label,
@@ -27,6 +27,8 @@ func CheckBox(th *Theme, checkBox *widget.Bool, label string) CheckBoxStyle {
 			uncheckedStateIcon: th.Icon.CheckBoxUnchecked,
 		},
 	}
+	c.checkable.Font.Typeface = th.Face
+	return c
 }
 
 // Layout updates the checkBox and displays it.