Adds DesignMode class

Provides a static boolean that can be checked by application authors whether their app is running in a tool considered to be a designer or previewer. Tool authors can set this value to true whenever the app is running in that context or not.

Resolves xamarin#1731

Xamarin.Forms.Core/DesignMode.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Xamarin.Forms.Core
+{
+    public static class DesignMode
+    {
+		/// <summary>
+		/// Enables you to detect whether your app is running in a visual designer or previewer.
+		/// Applications should not set this property. Instead, this is used by tools that will run an
+		/// application in a visual designer or previewer.
+		/// </summary>
+		/// <remarks>
+		/// A public setter to allow for third-party tools to enable this.
+		/// </remarks>
+		public static bool DesignModeEnabled { get; set; }
+	}
+}