Update C# wrapper to support both desktop and UWP projects

Summary:
The C# wrapper project has been changed to a NETStandard (1.1) PCL allowing it
to be consumed by any project targeting .NET4.5 or greater including .NETCore
and UWP projects... The C# wrapper uses P/Invoke to call into the Native Yoga
DLL...

The "Yoga" C++ project has been updated to support ARM builds...

Added the ability to generate nuget packages for the C# wrapper that supports
copying the native DLLs to the target output directory.

Closes: https://github.com/facebook/yoga/pull/283

Reviewed By: splhack, emilsjolander

Differential Revision: D4381455

Pulled By: JoelMarcey

fbshipit-source-id: e6dc4190caa824ac63d5d5ee2eb4c7914efbb00f

csharp/Windows/Facebook.Yoga.Universal.Tests/UnitTestApp.xaml.cs

@@ -0,0 +1,102 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Windows.ApplicationModel;
+using Windows.ApplicationModel.Activation;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+namespace Facebook.Yoga.Universal.Tests
+{
+    /// <summary>
+    /// Provides application-specific behavior to supplement the default Application class.
+    /// </summary>
+    sealed partial class App : Application
+    {
+        /// <summary>
+        /// Initializes the singleton application object.  This is the first line of authored code
+        /// executed, and as such is the logical equivalent of main() or WinMain().
+        /// </summary>
+        public App()
+        {
+            this.InitializeComponent();
+            this.Suspending += OnSuspending;
+        }
+
+        /// <summary>
+        /// Invoked when the application is launched normally by the end user.  Other entry points
+        /// will be used such as when the application is launched to open a specific file.
+        /// </summary>
+        /// <param name="e">Details about the launch request and process.</param>
+        protected override void OnLaunched(LaunchActivatedEventArgs e)
+        {
+
+#if DEBUG
+            if (System.Diagnostics.Debugger.IsAttached)
+            {
+                this.DebugSettings.EnableFrameRateCounter = true;
+            }
+#endif
+
+            Frame rootFrame = Window.Current.Content as Frame;
+
+            // Do not repeat app initialization when the Window already has content,
+            // just ensure that the window is active
+            if (rootFrame == null)
+            {
+                // Create a Frame to act as the navigation context and navigate to the first page
+                rootFrame = new Frame();
+
+                rootFrame.NavigationFailed += OnNavigationFailed;
+
+                if (e.PreviousExecutionState == ApplicationExecutionState.Terminated)
+                {
+                    //TODO: Load state from previously suspended application
+                }
+
+                // Place the frame in the current Window
+                Window.Current.Content = rootFrame;
+            }
+            
+            Microsoft.VisualStudio.TestPlatform.TestExecutor.UnitTestClient.CreateDefaultUI();
+
+            // Ensure the current window is active
+            Window.Current.Activate();
+
+            Microsoft.VisualStudio.TestPlatform.TestExecutor.UnitTestClient.Run(e.Arguments);
+        }
+
+        /// <summary>
+        /// Invoked when Navigation to a certain page fails
+        /// </summary>
+        /// <param name="sender">The Frame which failed navigation</param>
+        /// <param name="e">Details about the navigation failure</param>
+        void OnNavigationFailed(object sender, NavigationFailedEventArgs e)
+        {
+            throw new Exception("Failed to load Page " + e.SourcePageType.FullName);
+        }
+
+        /// <summary>
+        /// Invoked when application execution is being suspended.  Application state is saved
+        /// without knowing whether the application will be terminated or resumed with the contents
+        /// of memory still intact.
+        /// </summary>
+        /// <param name="sender">The source of the suspend request.</param>
+        /// <param name="e">Details about the suspend request.</param>
+        private void OnSuspending(object sender, SuspendingEventArgs e)
+        {
+            var deferral = e.SuspendingOperation.GetDeferral();
+            //TODO: Save application state and stop any background activity
+            deferral.Complete();
+        }
+    }
+}