Register export plugins, breaking cycles...

... as we did last relase for import; making several header files unnecessary.

This breaks up a strongly connected component of 9, which was the largest
remaining.  Now the largest remaining is 5.

src/export/ExportOGG.cpp

@@ -18,7 +18,6 @@
 #include "../Audacity.h" // for USE_* macros
 
 #ifdef USE_LIBVORBIS
-#include "ExportOGG.h"
 
 #include "Export.h"
 
@@ -143,6 +142,8 @@ public:
                const Tags *metadata = NULL,
                int subformat = 0) override;
 
+   virtual unsigned SequenceNumber() const override { return 30; }
+
 private:
 
    bool FillComment(AudacityProject *project, vorbis_comment *comment, const Tags *metadata);
@@ -397,10 +398,8 @@ bool ExportOGG::FillComment(AudacityProject *project, vorbis_comment *comment, c
    return true;
 }
 
-std::unique_ptr<ExportPlugin> New_ExportOGG()
-{
-   return std::make_unique<ExportOGG>();
-}
+static Exporter::RegisteredExportPlugin
+sRegisteredPlugin{ []{ return std::make_unique< ExportOGG >(); } };
 
 #endif // USE_LIBVORBIS