Revised layout interface

object.go

@@ -179,9 +179,17 @@ type ContainerBox interface {
 	SetLayout (Layout)
 }
 
+// LayoutHints are passed to a layout to tell it how to arrange child boxes.
+type LayoutHints struct {
+	Min image.Point
+	Max *image.Point
+	Gap image.Point
+}
+
 // Layout can be given to a ContainerBox to arrange child objects.
 type Layout interface {
-	Arrange (image.Rectangle, Gap, []Box)
+	MinimumSize (LayoutHints, []Box) image.Point
+	Arrange     (LayoutHints, []Box)
 }
 
 // Window is an operating system window. It can contain one object.