Improve IImageViewBackend.SetImage API

 - Added docs
 - Added argument exceptions
 - Renamed 'imageBackend' to 'nativeImage' to better reflect the actual
   object being passed in.

Xwt.Gtk/Xwt.GtkBackend/ImageViewBackend.cs

@@ -41,9 +41,16 @@ public override void Initialize ()
 			set { base.Widget = value; }
 		}
 
-		public void SetImage (object imageBackend)
+		public void SetImage (object nativeImage)
 		{
-			Widget.Pixbuf = (Gdk.Pixbuf)imageBackend;
+			if (nativeImage == null)
+				throw new ArgumentNullException ("nativeImage");
+
+			Gdk.Pixbuf pbuf = nativeImage as Gdk.Pixbuf;
+			if (pbuf == null)
+				throw new ArgumentException ("nativeImage is not of the expected type", "nativeImage");
+
+			Widget.Pixbuf = pbuf;
 		}
 	}
 }

Xwt.Mac/Xwt.Mac/ImageViewBackend.cs

@@ -42,9 +42,16 @@ public override void Initialize ()
 			Widget.SizeToFit ();
 		}
 
-		public void SetImage (object imageBackend)
+		public void SetImage (object nativeImage)
 		{
-			Widget.Image = (NSImage) imageBackend;
+			if (nativeImage == null)
+				throw new ArgumentNullException ("nativeImage");
+
+			NSImage image = nativeImage as NSImage;
+			if (image == null)
+				throw new ArgumentException ("nativeImage is not of the expected type", "nativeImage");
+
+			Widget.Image = image;
 			Widget.SetFrameSize (Widget.Image.Size);
 		}
 	}

Xwt/Xwt.Backends/IImageViewBackend.cs

@@ -29,7 +29,13 @@ namespace Xwt.Backends
 {
 	public interface IImageViewBackend: IWidgetBackend
 	{
-		void SetImage (object imageBackend);
+		/// <summary>
+		/// Sets the image view to use the supplied image backend.
+		/// </summary>
+		/// <param name="nativeImage">A native image widget.</param>
+		/// <exception cref="ArgumentNullException"><paramref name="nativeImage"/> is <c>null</c>.</exception>
+		/// <exception cref="ArgumentException"><paramref name="nativeImage"/> is not of the expected type.</exception>
+		void SetImage (object nativeImage);
 	}
 }