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/theme.go

@@ -7,6 +7,7 @@ import (
 
 	"golang.org/x/exp/shiny/materialdesign/icons"
 
+	"gioui.org/font"
 	"gioui.org/text"
 	"gioui.org/unit"
 	"gioui.org/widget"
@@ -41,6 +42,8 @@ type Theme struct {
 		RadioChecked      *widget.Icon
 		RadioUnchecked    *widget.Icon
 	}
+	// Face selects the default typeface for text.
+	Face font.Typeface
 
 	// FingerSize is the minimum touch target size.
 	FingerSize unit.Dp