widget/material: manage widget colors with Palette type

This introduces a new material.Palette type that captures the color information
necessary to render a widget. This type is embedded in the material.Theme to
make it easier to swap to a different palette for part of the UI by reassinging
the Palette field.

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

widget/material/label.go

@@ -66,7 +66,7 @@ func Caption(th *Theme, txt string) LabelStyle {
 func Label(th *Theme, size unit.Value, txt string) LabelStyle {
 	return LabelStyle{
 		Text:     txt,
-		Color:    th.Color.Text,
+		Color:    th.Palette.Fg,
 		TextSize: size,
 		shaper:   th.Shaper,
 	}