implement enum for types, implement OpenDialog from Dialog-API

ElectronNET.API/Entities/WebPreferences.cs

@@ -1,4 +1,7 @@
-namespace ElectronNET.API.Entities
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+
+namespace ElectronNET.API.Entities
 {
     public class WebPreferences
     {
@@ -121,6 +124,11 @@
         /// </summary>
         public string DisableBlinkFeatures { get; set; }
 
+        /// <summary>
+        /// Sets the default font for the font-family.
+        /// </summary>
+        public DefaultFontFamily DefaultFontFamily { get; set; }
+
         /// <summary>
         /// Defaults to 16.
         /// </summary>