Add window behavior to box

object.go

@@ -104,6 +104,8 @@ type Object interface {
 
 type Box interface {
 	Object
+
+	Window         () Window
 	Bounds         () image.Rectangle
 	InnerBounds    () image.Rectangle
 	SetBounds      (image.Rectangle)
@@ -118,8 +120,8 @@ type Box interface {
 	SetFocusable (bool)
 	
 	Focused       () bool
-	Modifiers     (func ()) input.Modifiers
-	MousePosition (func ()) image.Point
+	Modifiers     () input.Modifiers
+	MousePosition () image.Point
 
 	OnFocusEnter  (func ()) event.Cookie
 	OnFocusLeave  (func ()) event.Cookie