Use wxSpinCtrl best size in wxGenericCalendarCtrl::DoGetBestSize

This fixes the wxGenericCalendarCtrl on wxGTK3 where the SpinCtrl is
much wider than on other platforms and should also ensure that the
control is always big enough in all locales under Mac.

Closes https://github.com/wxWidgets/wxWidgets/pull/2604

Closes #11444.

samples/calendar/calendar.cpp

@@ -397,7 +397,7 @@ bool MyApp::OnInit()
 
     // Create the main application window
     MyFrame *frame = new MyFrame("Calendar wxWidgets sample"
-                                 ,wxPoint(50, 50), wxSize(450, 340)
+                                 ,wxPoint(50, 50), wxSize(460, 340)
                                  );
 
     frame->Show(true);