Add "Empty" method to Hierarchy

internal/system/hierarchy.go

@@ -73,6 +73,11 @@ func (this *Hierarchy) SetRoot (root tomo.Box) {
 	this.minimumClean = false
 }
 
+// Empty returns whether or not the root of the Hierarchy is nil.
+func (this *Hierarchy) Empty () bool {
+	return this.root == nil
+}
+
 // SetCanvas sets the held canvas of the Hierarchy that all boxes within it will
 // draw to. The Hierarchy will use the canvas.Canvas's bounds to lay itself out.
 func (this *Hierarchy) SetCanvas (can canvas.Canvas) {