Handle applicationShouldTerminate on OSX.

Added some new plumbing to facilitate this:

- A managed event is raised `IPlatformLifetimeEventsImpl.ShutdownRequested`
- This event is subscribed from `ClassicDesktopStyleApplicationLifetime`
- Which tries to close all windows
- And returns false to cancel the OS-level close if not all windows could be closed

native/Avalonia.Native/src/OSX/app.mm

@@ -50,6 +50,12 @@ ComPtr<IAvnApplicationEvents> _events;
     
     _events->FilesOpened(array);
 }
+
+- (NSApplicationTerminateReply)applicationShouldTerminate:(NSApplication *)sender
+{
+    return _events->TryShutdown() ? NSTerminateNow : NSTerminateCancel;
+}
+
 @end
 
 @interface AvnApplication : NSApplication

src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Linq;
 using System.Threading;
 using Avalonia.Controls;
@@ -42,7 +43,7 @@ namespace Avalonia.Controls.ApplicationLifetimes
                     "Can not have multiple active ClassicDesktopStyleApplicationLifetime instances and the previously created one was not disposed");
             _activeLifetime = this;
         }
-        
+
         /// <inheritdoc/>
         public event EventHandler<ControlledApplicationLifetimeStartupEventArgs> Startup;
         /// <inheritdoc/>
@@ -111,6 +112,11 @@ namespace Avalonia.Controls.ApplicationLifetimes
                 ((IApplicationPlatformEvents)Application.Current).RaiseUrlsOpened(args);
             }
 
+            var lifetimeEvents = AvaloniaLocator.Current.GetService<IPlatformLifetimeEventsImpl>(); 
+
+            if (lifetimeEvents != null)
+                lifetimeEvents.ShutdownRequested += ShutdownRequested;
+
             _cts = new CancellationTokenSource();
             MainWindow?.Show();
             Dispatcher.UIThread.MainLoop(_cts.Token);
@@ -123,6 +129,20 @@ namespace Avalonia.Controls.ApplicationLifetimes
             if (_activeLifetime == this)
                 _activeLifetime = null;
         }
+        
+        private void ShutdownRequested(object sender, CancelEventArgs e)
+        {
+            foreach (var w in Windows.ToArray())
+                w.Close();
+            if (Windows.Count > 0)
+                e.Cancel = true;
+        }
+
+        private bool TryCloseAllWindows()
+        {
+
+            return Windows.Count == 0;
+        }
     }
     
     public class ClassicDesktopStyleApplicationLifetimeOptions

src/Avalonia.Controls/Platform/IPlatformLifetimeEventsImpl.cs

@@ -0,0 +1,16 @@
+using System;
+using System.ComponentModel;
+
+namespace Avalonia.Platform
+{
+    public interface IPlatformLifetimeEventsImpl
+    {
+        /// <summary>
+        /// Raised by the platform when a shutdown is requested.
+        /// </summary>
+        /// <remarks>
+        /// Raised on on OSX via the Quit menu or right-clicking on the application icon and selecting Quit.
+        /// </remarks>
+        event EventHandler<CancelEventArgs> ShutdownRequested;
+    }
+}

src/Avalonia.Native/AvaloniaNativeApplicationPlatform.cs

@@ -1,14 +1,25 @@
 using System;
+using System.ComponentModel;
 using Avalonia.Native.Interop;
 using Avalonia.Platform;
 
 namespace Avalonia.Native
 {
-    internal class AvaloniaNativeApplicationPlatform : CallbackBase, IAvnApplicationEvents
+    internal class AvaloniaNativeApplicationPlatform : CallbackBase, IAvnApplicationEvents, IPlatformLifetimeEventsImpl
     {
+        public event EventHandler<CancelEventArgs> ShutdownRequested;
+        
         void IAvnApplicationEvents.FilesOpened(IAvnStringArray urls)
         {
             ((IApplicationPlatformEvents)Application.Current).RaiseUrlsOpened(urls.ToStringArray());
         }
+
+        public int TryShutdown()
+        {
+            if (ShutdownRequested is null) return 1;
+            var e = new CancelEventArgs();
+            ShutdownRequested(this, e);
+            return (!e.Cancel).AsComBool();
+        }
     }
 }

src/Avalonia.Native/AvaloniaNativePlatform.cs

@@ -111,7 +111,8 @@ namespace Avalonia.Native
                 .Bind<ISystemDialogImpl>().ToConstant(new SystemDialogs(_factory.CreateSystemDialogs()))
                 .Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration(KeyModifiers.Meta))
                 .Bind<IMountedVolumeInfoProvider>().ToConstant(new MacOSMountedVolumeInfoProvider())
-                .Bind<IPlatformDragSource>().ToConstant(new AvaloniaNativeDragSource(_factory));
+                .Bind<IPlatformDragSource>().ToConstant(new AvaloniaNativeDragSource(_factory))
+                .Bind<IPlatformLifetimeEventsImpl>().ToConstant(applicationPlatform);
 
             if (_options.UseGpu)
             {

src/Avalonia.Native/avn.idl

@@ -732,4 +732,5 @@ interface IAvnNativeControlHostTopLevelAttachment : IUnknown
 interface IAvnApplicationEvents : IUnknown
 {
      void FilesOpened (IAvnStringArray* urls);
+     bool TryShutdown();
 }