Subsystems can now be selected via Application extension methods.

src/Gtk/Perspex.Cairo/CairoPlatform.cs

@@ -7,6 +7,18 @@ using Perspex.Cairo.Media.Imaging;
 using Perspex.Media;
 using Perspex.Platform;
 
+namespace Perspex
+{
+	public static class GtkApplicationExtensions
+	{
+		public static AppT UseCairo<AppT>(this AppT app) where AppT : Application
+		{
+			Perspex.Cairo.CairoPlatform.Initialize();
+			return app;
+		}
+	}
+}
+
 namespace Perspex.Cairo
 {
     using System.IO;

src/Gtk/Perspex.Gtk/GtkPlatform.cs

@@ -10,6 +10,18 @@ using Perspex.Input;
 using Perspex.Platform;
 using Perspex.Shared.PlatformSupport;
 
+namespace Perspex
+{
+	public static class GtkApplicationExtensions
+	{
+		public static AppT UseGtkSubsystem<AppT>(this AppT app) where AppT : Application
+		{
+			Perspex.Gtk.GtkPlatform.Initialize();
+			return app;
+		}
+	}
+}
+
 namespace Perspex.Gtk
 {
     using Gtk = global::Gtk;

src/Skia/Perspex.Skia/SkiaPlatform.cs

@@ -3,6 +3,18 @@ using System.Collections.Generic;
 using System.Text;
 using Perspex.Platform;
 
+namespace Perspex
+{
+	public static class SkiaApplicationExtensions
+	{
+		public static AppT UseSkia<AppT>(this AppT app) where AppT : Application
+		{
+			Perspex.Skia.SkiaPlatform.Initialize();
+			return app;
+		}
+	}
+}
+
 namespace Perspex.Skia
 {
     public static class SkiaPlatform

src/Windows/Perspex.Direct2D1/Direct2D1Platform.cs

@@ -7,6 +7,18 @@ using Perspex.Direct2D1.Media;
 using Perspex.Media;
 using Perspex.Platform;
 
+namespace Perspex
+{
+	public static class Direct2DApplicationExtensions
+	{
+		public static AppT UseDirect2D<AppT>(this AppT app) where AppT : Application
+		{
+			Perspex.Direct2D1.Direct2D1Platform.Initialize();
+			return app;
+		}
+	}
+}
+
 namespace Perspex.Direct2D1
 {
     public class Direct2D1Platform : IPlatformRenderInterface

src/Windows/Perspex.Win32/Win32Platform.cs

@@ -16,6 +16,18 @@ using Perspex.Shared.PlatformSupport;
 using Perspex.Win32.Input;
 using Perspex.Win32.Interop;
 
+namespace Perspex
+{
+	public static class Win32ApplicationExtensions
+	{
+		public static AppT UseWin32Subsystem<AppT>(this AppT app) where AppT : Application
+		{
+			Perspex.Win32.Win32Platform.Initialize();
+			return app;
+		}
+	}
+}
+
 namespace Perspex.Win32
 {
     public class Win32Platform : IPlatformThreadingInterface, IPlatformSettings, IWindowingPlatform