export: made progress on headless export

Still incomplete and unusable, but less so than before.

app/codec/encoder.h

@@ -23,6 +23,7 @@
 
 #include <memory>
 #include <QString>
+#include <QXmlStreamWriter>
 
 #include "codec/exportcodec.h"
 #include "codec/exportformat.h"
@@ -69,6 +70,8 @@ public:
   const rational& GetExportLength() const;
   void SetExportLength(const rational& GetExportLength);
 
+  virtual void Save(QXmlStreamWriter* writer) const;
+
 private:
   QString filename_;