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/button.go

@@ -51,7 +51,7 @@ type IconButtonStyle struct {
 }
 
 func Button(th *Theme, button *widget.Clickable, txt string) ButtonStyle {
-	return ButtonStyle{
+	b := ButtonStyle{
 		Text:         txt,
 		Color:        th.Palette.ContrastFg,
 		CornerRadius: 4,
@@ -64,6 +64,8 @@ func Button(th *Theme, button *widget.Clickable, txt string) ButtonStyle {
 		Button: button,
 		shaper: th.Shaper,
 	}
+	b.Font.Typeface = th.Face
+	return b
 }
 
 func ButtonLayout(th *Theme, button *widget.Clickable) ButtonLayoutStyle {