layout: replace Align with a Layout method on Direction

It's one less type (Align) and shorter:

Before:

	layout.Align(layout.Center).Layout(...)

After

	layout.Center.Layout(...)

It is also safer: since `layout.Align(...)` was a casting operation,
the Go compiler would not complain about an incompatible constant.

For example, the widget/material package contained a wrong cast:

	layout.Align(layout.Start)

which should have been

	layout.Align(layout.W)

After this change, attempting `layout.Start.Layout(...)` result
in a compile error.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/material/button.go

@@ -80,7 +80,7 @@ func (b Button) Layout(gtx *layout.Context, button *widget.Button) {
 		layout.Stacked(func() {
 			gtx.Constraints.Width.Min = hmin
 			gtx.Constraints.Height.Min = vmin
-			layout.Align(layout.Center).Layout(gtx, func() {
+			layout.Center.Layout(gtx, func() {
 				layout.Inset{Top: unit.Dp(10), Bottom: unit.Dp(10), Left: unit.Dp(12), Right: unit.Dp(12)}.Layout(gtx, func() {
 					paint.ColorOp{Color: col}.Add(gtx.Ops)
 					widget.Label{}.Layout(gtx, b.shaper, b.Font, b.Text)