Make message boxes more platform independent with a wrapper

Instead  of  using the  Qt  QMessageBox  class  directly a  new  class
MessageBox  (MessageBox.hpp)  has been  added  to  deal with  platform
independence issues like the title not being shown on Mac OS X.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6861 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

SampleDownloader.cpp

@@ -4,10 +4,10 @@
 #include <QSettings>
 #include <QtWidgets>
 
+#include "pimpl_impl.hpp"
 #include "SettingsGroup.hpp"
 #include "SampleDownloader/Directory.hpp"
-
-#include "pimpl_impl.hpp"
+#include "MessageBox.hpp"
 
 #include "moc_SampleDownloader.cpp"
 
@@ -124,7 +124,7 @@ SampleDownloader::impl::impl (QSettings * settings
         }
       else
         {
-          QMessageBox::warning (this, "Input Error", "Invalid URL format");
+          MessageBox::warning_message (this, tr ("Input Error"), tr ("Invalid URL format"));
         }
     });
 }