Redid the entity system a bit to make it more reliable

Now it supports things like parenting elements before they are
added to a window and elements no longer have to constantly check
for a nil entity

backend.go

@@ -17,6 +17,9 @@ type Backend interface {
 	// possible. This method must be safe to call from other threads.
 	Do (callback func ())
 
+	// NewEntity creates a new entity for the specified element.
+	NewEntity (owner Element) Entity
+
 	// NewWindow creates a new window within the specified bounding
 	// rectangle. The position on screen may be overridden by the backend or
 	// operating system.

backends/x/entity.go

@@ -19,29 +19,46 @@ type entity struct {
 	isContainer   bool
 }
 
-func bind (parent *entity, window *window, element tomo.Element) *entity {
+func (backend *Backend) NewEntity (owner tomo.Element) tomo.Entity {
-	entity := &entity {
+	entity := &entity { element: owner }
-		window:  window,
+	if _, ok := owner.(tomo.Container); ok {
-		parent:  parent,
-		element: element,
-	}
-	entity.Invalidate()
-	if _, ok := element.(tomo.Container); ok {
 		entity.isContainer = true
 		entity.InvalidateLayout()
 	}
-
-	element.Bind(entity)
 	return entity
 }
 
-func (entity *entity) unbind () {
+func (ent *entity) unlink () {
-	entity.element.Bind(nil)
+	ent.propagate (func (child *entity) bool {
-	for _, childEntity := range entity.children {
+		child.window = nil
-		childEntity.unbind()
+		delete(ent.window.system.drawingInvalid, child)
+		return true
+	})
+	
+	delete(ent.window.system.drawingInvalid, ent)
+	ent.parent = nil
+	ent.window = nil
+}
+
+func (entity *entity) link (parent *entity) {
+	entity.parent = parent
+	if parent.window != nil {
+		entity.setWindow(parent.window)
 	}
 }
 
+func (ent *entity) setWindow (window *window) {
+	ent.window = window
+	ent.Invalidate()
+	ent.InvalidateLayout()
+	ent.propagate (func (child *entity) bool {
+		child.window = window
+		ent.Invalidate()
+		ent.InvalidateLayout()
+		return true
+	})
+}
+
 func (entity *entity) propagate (callback func (*entity) bool) {
 	for _, child := range entity.children {
 		if callback(child) { break }