优化更新代码，添加界面功能并整合

WPFluent/Controls/TreeView/TreeViewItem.cs

@@ -0,0 +1,27 @@
+
+
+
+// ReSharper disable once CheckNamespace
+namespace WPFluent.Controls;
+
+/// <summary>
+/// Extended <see cref="System.Windows.Controls.TreeViewItem"/> with <see cref="SymbolRegular"/> properties.
+/// </summary>
+public partial class TreeViewItem : System.Windows.Controls.TreeViewItem
+{
+    /// <summary>
+    /// Identifies the <see cref="Icon"/> dependency property.
+    /// </summary>
+    public static readonly DependencyProperty IconProperty = DependencyProperty.Register(
+        nameof(Icon),
+        typeof(IconElement),
+        typeof(TreeViewItem),
+        new PropertyMetadata(null));
+
+    /// <summary>
+    /// Gets or sets displayed <see cref="IconElement"/>.
+    /// </summary>
+    [Bindable(true)]
+    [Category("Appearance")]
+    public IconElement? Icon { get => (IconElement?)GetValue(IconProperty); set => SetValue(IconProperty, value); }
+}