Added missing Converter.

src/Avalonia.Controls/Converters/MarginMultiplierConverter.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace Avalonia.Controls.Converters
+{
+    public class MarginMultiplierConverter : IValueConverter
+    {
+        public double Indent { get; set; }
+
+        public bool Left { get; set; } = false;
+
+        public bool Top { get; set; } = false;
+
+        public bool Right { get; set; } = false;
+
+        public bool Bottom { get; set; } = false;
+
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (!(value is int depth))
+                return new Thickness(0);
+
+            return new Thickness(Left ? Indent * depth : 0, Top ? Indent * depth : 0, Right ? Indent * depth : 0, Bottom ? Indent * depth : 0);
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new System.NotImplementedException();
+        }
+    }
+}