优化更新代码，添加界面功能并整合

WPFluent/Controls/TextBox/ElementPlacement.cs

@@ -0,0 +1,20 @@
+
+
+
+namespace WPFluent.Controls;
+
+/// <summary>
+/// Decides where to put the element.
+/// </summary>
+public enum ElementPlacement
+{
+    /// <summary>
+    /// Puts the control element on the left.
+    /// </summary>
+    Left,
+
+    /// <summary>
+    /// Puts the control element on the right.
+    /// </summary>
+    Right,
+}