优化更新代码，添加界面功能并整合

WPFluent/Converters/TreeLevelToIndentConverter.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace WPFluent.Converters;
+
+[ValueConversion(typeof(int), typeof(Thickness))]
+internal sealed class TreeLevelToIndentConverter : IValueConverter
+{
+    private const double IndentSize = 19d;
+
+    public object? Convert(object? value, Type type, object? parameter, CultureInfo culture)
+    {
+        return new Thickness((int)value! * IndentSize, 0d, 0d, 0d);
+    }
+
+    public object? ConvertBack(object? value, Type type, object? parameter, CultureInfo culture)
+    {
+        throw new NotSupportedException();
+    }
+}