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>

layout/example_test.go

@@ -30,13 +30,12 @@ func ExampleInset() {
 	// (70,70)
 }
 
-func ExampleAlign() {
+func ExampleDirection() {
 	gtx := new(layout.Context)
 	// Rigid constraints with both minimum and maximum set.
 	gtx.Reset(nil, image.Point{X: 100, Y: 100})
 
-	align := layout.Align(layout.Center)
-	align.Layout(gtx, func() {
+	layout.Center.Layout(gtx, func() {
 		// Lay out a 50x50 sized widget.
 		layoutWidget(gtx, 50, 50)
 		fmt.Println(gtx.Dimensions.Size)