sub folder for element containers + horizontal panel + shorthands for horizontal and vertical panel

Source/Editor/CustomEditors/Elements/Container/TreeNodeElement.cs

@@ -0,0 +1,44 @@
+// Copyright (c) 2012-2021 Wojciech Figat. All rights reserved.
+
+using FlaxEditor.GUI.Tree;
+using FlaxEngine.GUI;
+
+namespace FlaxEditor.CustomEditors.Elements
+{
+    /// <summary>
+    /// Tree nodes elements.
+    /// </summary>
+    public interface ITreeElement
+    {
+        /// <summary>
+        /// Adds new tree node element.
+        /// </summary>
+        /// <param name="text">The node name (title text).</param>
+        /// <returns>The created element.</returns>
+        TreeNodeElement Node(string text);
+    }
+
+    /// <summary>
+    /// The tree structure node element.
+    /// </summary>
+    /// <seealso cref="FlaxEditor.CustomEditors.LayoutElementsContainer" />
+    public class TreeNodeElement : LayoutElementsContainer, ITreeElement
+    {
+        /// <summary>
+        /// The tree node control.
+        /// </summary>
+        public readonly TreeNode TreeNode = new TreeNode(false);
+
+        /// <inheritdoc />
+        public override ContainerControl ContainerControl => TreeNode;
+
+        /// <inheritdoc />
+        public TreeNodeElement Node(string text)
+        {
+            TreeNodeElement element = new TreeNodeElement();
+            element.TreeNode.Text = text;
+            OnAddElement(element);
+            return element;
+        }
+    }
+}