Add layout components 001-020 with demos and behavior tests

layout/layers.go

@@ -0,0 +1,19 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// Layers stacks layers in a box; the first sizes it.
+func Layers(c *ui.Context, layers ...func()) *ui.Element {
+	if len(layers) == 0 {
+		panic("layout: Layers needs a layer")
+	}
+	return ui.Box(c).Children(func() {
+		for i, l := range layers {
+			b := ui.Box(c).Fill()
+			if i > 0 {
+				b.Absolute().Top(0).Left(0)
+			}
+			b.Children(l)
+		}
+	})
+}