Add **UI navigation** system

#574

Source/Engine/UI/GUI/Enums.cs

@@ -167,4 +167,40 @@ namespace FlaxEngine.GUI
         /// </summary>
         Vertical = 1,
     }
+
+    /// <summary>
+    /// The navigation directions in the user interface layout.
+    /// </summary>
+    public enum NavDirection
+    {
+        /// <summary>
+        /// No direction to skip navigation.
+        /// </summary>
+        None,
+
+        /// <summary>
+        /// The up direction.
+        /// </summary>
+        Up,
+
+        /// <summary>
+        /// The down direction.
+        /// </summary>
+        Down,
+
+        /// <summary>
+        /// The left direction.
+        /// </summary>
+        Left,
+
+        /// <summary>
+        /// The right direction.
+        /// </summary>
+        Right,
+
+        /// <summary>
+        /// The next item (right with layout wrapping).
+        /// </summary>
+        Next,
+    }
 }