support for .NET framework 3.5

Still have bugs

Net35/HandyControlDemo_Net35/Data/AppConfig.cs

@@ -0,0 +1,14 @@
+using System;
+using HandyControl.Data;
+
+namespace HandyControlDemo.Data
+{
+    internal class AppConfig
+    {
+        public static string SavePath = $"{AppDomain.CurrentDomain.BaseDirectory}AppConfig.json";
+
+        public string Lang { get; set; } = "zh-cn";
+
+        public SkinType Skin { get; set; }
+    }
+}