refactor so that this can be set on X11Options as well

src/Avalonia.X11/X11Platform.cs

@@ -81,7 +81,7 @@ namespace Avalonia.X11
                 .Bind<IPlatformIconLoader>().ToConstant(new X11IconLoader(Info))
                 .Bind<ISystemDialogImpl>().ToConstant(new GtkSystemDialog())
                 .Bind<IMountedVolumeInfoProvider>().ToConstant(new LinuxMountedVolumeInfoProvider())
-                .Bind<IPlatformLifetimeEventsImpl>().ToConstant(new X11PlatformLifetimeEvents());
+                .Bind<IPlatformLifetimeEventsImpl>().ToConstant(new X11PlatformLifetimeEvents(this));
             
             X11Screens = Avalonia.X11.X11Screens.Init(this);
             Screens = new X11Screens(X11Screens);
@@ -231,7 +231,19 @@ namespace Avalonia
         /// on their input devices by using sequences of characters or mouse operations that are natively available on their input devices.
         /// </remarks>
         public bool? EnableIme { get; set; }
-
+        
+        /// <summary>
+        /// Determines whether to enable support for the
+        /// X Session Management Protocol.
+        /// </summary>
+        /// <remarks>
+        /// X Session Management Protocol is a standard implemented on most
+        /// Linux systems that uses Xorg. This enables apps to control how they
+        /// can control and/or cancel the pending shutdown requested by the user.
+        /// </remarks>
+        public bool EnableSessionManagement { get; set; } = 
+            Environment.GetEnvironmentVariable("AVALONIA_X11_USE_SESSION_MANAGEMENT") != "0";
+        
         public IList<GlVersion> GlProfiles { get; set; } = new List<GlVersion>
         {
             new GlVersion(GlProfileType.OpenGL, 4, 0),

src/Avalonia.X11/X11PlatformLifetimeEvents.cs

@@ -11,8 +11,9 @@ using Avalonia.Threading;
 
 namespace Avalonia.X11
 {
-    public unsafe class X11PlatformLifetimeEvents : IDisposable, IPlatformLifetimeEventsImpl
+    internal unsafe class X11PlatformLifetimeEvents : IDisposable, IPlatformLifetimeEventsImpl
     {
+        private readonly AvaloniaX11Platform _platform;
         private const ulong SmcSaveYourselfProcMask = 1L;
         private const ulong SmcDieProcMask = 2L;
         private const ulong SmcSaveCompleteProcMask = 4L;
@@ -34,9 +35,6 @@ namespace Avalonia.X11
         private static readonly ICELib.IceIOErrorHandler s_iceIoErrorHandlerDelegate = StaticIceIOErrorHandler;
         private static readonly SMLib.IceWatchProc s_iceWatchProcDelegate = IceWatchHandler;
 
-        private bool AllowShutdownCancellation =>
-            Environment.GetEnvironmentVariable("AVALONIA_X11_USE_SESSION_MANAGER") != "0";
-
         private static SMLib.SmcCallbacks s_callbacks = new SMLib.SmcCallbacks()
         {
             ShutdownCancelled = Marshal.GetFunctionPointerForDelegate(s_shutdownCancelledDelegate),
@@ -51,8 +49,10 @@ namespace Avalonia.X11
 
         private bool _saveYourselfPhase;
 
-        public X11PlatformLifetimeEvents()
+        internal X11PlatformLifetimeEvents(AvaloniaX11Platform platform)
         {
+            _platform = platform;
+            
             if (ICELib.IceAddConnectionWatch(
                     Marshal.GetFunctionPointerForDelegate(s_iceWatchProcDelegate),
                     IntPtr.Zero) == 0)
@@ -229,7 +229,7 @@ namespace Avalonia.X11
         {
             var e = new ShutdownRequestedEventArgs();
 
-            if (AllowShutdownCancellation)
+            if (_platform.Options?.EnableSessionManagement ?? false)
             {
                 ShutdownRequested?.Invoke(this, e);
             }