export: fixed many export formats including still image and image sequence export

Still incomplete, but does a lot towards #1345

app/common/qtutils.h

@@ -29,6 +29,7 @@
 
 #include <QFontMetrics>
 #include <QFrame>
+#include <QMessageBox>
 
 #include "common/define.h"
 
@@ -47,6 +48,8 @@ public:
 
   static QFrame* CreateHorizontalLine();
 
+  static int MessageBox(QWidget *parent, QMessageBox::Icon icon, const QString& title, const QString& message, QMessageBox::StandardButtons buttons = QMessageBox::Ok);
+
 };
 
 }