Get Proxy Server for Web Client asynchronously

This prevents WebRequest.GetSystemWebProxy() from blocking UI

ILSpy/AboutPage.cs

@@ -179,6 +179,7 @@ namespace ICSharpCode.ILSpy
 		static Task<AvailableVersionInfo> GetLatestVersionAsync()
 		{
 			var tcs = new TaskCompletionSource<AvailableVersionInfo>();
+			new Action(() => {
 				WebClient wc = new WebClient();
 				IWebProxy systemWebProxy = WebRequest.GetSystemWebProxy();
 				systemWebProxy.Credentials = CredentialCache.DefaultCredentials;
@@ -203,6 +204,7 @@ namespace ICSharpCode.ILSpy
 					}
 				};
 				wc.DownloadDataAsync(UpdateUrl);
+			}).BeginInvoke(null, null);
 			return tcs.Task;
 		}