Added door.Screen (a group of panels).

door/screen.go

@@ -0,0 +1,25 @@
+package door
+
+type Screen struct {
+	Panels []Panel
+}
+
+func (s *Screen) AddPanel(panel Panel) {
+	s.Panels = append(s.Panels, panel)
+}
+
+func (s *Screen) Output() string {
+	var result string
+	for idx, _ := range s.Panels {
+		result += s.Panels[idx].Output()
+	}
+	return result
+}
+
+func (s *Screen) Update() string {
+	var result string
+	for idx, _ := range s.Panels {
+		result += s.Panels[idx].Update()
+	}
+	return result
+}