Make wxProgressDialog::Fit() work in native MSW version

This method is supposed to adjust the dialog size to its contents and
while the dialog increases automatically when using native
implementation under MSW, it doesn't shrink back on its own and so it's
still useful to allow Fit() to do it.

Update the sample to test Fit() too.

samples/dialogs/dialogs.cpp

@@ -360,6 +360,11 @@ bool MyApp::OnInit()
                 case 30:
                     msg = "Back to brevity";
                     break;
+
+                case 80:
+                    msg = "Back and adjusted";
+                    dlg.Fit();
+                    break;
             }
 
             if ( !dlg.Update(i, msg) )