Register export plugins, as we did for import, breaking cycles...

... and making several header files unnecessary

src/export/ExportFFmpeg.cpp

@@ -20,7 +20,6 @@ function.
 
 
 #include "../Audacity.h"   // keep ffmpeg before wx because they interact // for USE_* macros
-#include "ExportFFmpeg.h"
 
 #include "../FFmpeg.h"     // and Audacity.h before FFmpeg for config*.h
 
@@ -151,6 +150,8 @@ public:
       const Tags *metadata = NULL,
       int subformat = 0) override;
 
+   virtual unsigned SequenceNumber() const override { return 70; }
+
 private:
 
    AVOutputFormat  *   mEncFormatDesc{};       // describes our output file to libavformat
@@ -1095,10 +1096,8 @@ wxWindow *ExportFFmpeg::OptionsCreate(wxWindow *parent, int format)
    return ExportPlugin::OptionsCreate(parent, format);
 }
 
-std::unique_ptr<ExportPlugin> New_ExportFFmpeg()
-{
-   return std::make_unique<ExportFFmpeg>();
-}
+static Exporter::RegisteredExportPlugin
+sRegisteredPlugin{ []{ return std::make_unique< ExportFFmpeg >(); } };
 
 #endif