fix: CustomElementの子要素が親を持たない不具合を修正

Promete/Elements/Container.cs

@@ -44,7 +44,6 @@ public void RemoveAt(int index)
 
 	public void Add(ElementBase item)
 	{
-		children.Add(item);
 		item.Parent = this;
 	}
 

Promete/Elements/CustomElement.cs

@@ -12,6 +12,7 @@ internal override void Update()
 		base.Update();
 		for (var i = 0; i < children.Count; i++)
 		{
+			children[i].Parent = this;
 			children[i].Update();
 		}
 	}

Promete/Elements/ElementBase.cs

@@ -48,7 +48,7 @@ public int Height
 	public Vector AbsoluteScale => Parent == null ? Scale : Scale * Parent.AbsoluteScale;
 	public float AbsoluteAngle => Parent == null ? Angle : Angle + Parent.AbsoluteAngle;
 
-	public Container? Parent { get; internal set; }
+	public ElementBase? Parent { get; internal set; }
 
 	private readonly List<Component> components = [];