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/FileNode.cpp

@@ -4,9 +4,9 @@
 #include <QUrl>
 #include <QDir>
 #include <QFileInfo>
-#include <QMessageBox>
 
 #include "Directory.hpp"
+#include "MessageBox.hpp"
 
 FileNode::FileNode (QTreeWidgetItem * parent
                     , QNetworkAccessManager * network_manager
@@ -26,7 +26,7 @@ FileNode::FileNode (QTreeWidgetItem * parent
 
 void FileNode::error (QString const& title, QString const& message)
 {
-  QMessageBox::warning (treeWidget (), title, message);
+  MessageBox::warning_message (treeWidget (), title, message);
 }
 
 bool FileNode::sync (bool local)