Dispatch read of top-level project XML tags with a table of functions...

... which makes Project.cpp a bit less dependent on some details of other
classes

This puts Tags.cpp back into the big strongly connected component of the
dependency graph.  That will be remedied later when Project.cpp becomes a
low-level file

src/ProjectFileIORegistry.cpp

@@ -0,0 +1,44 @@
+/**********************************************************************
+
+  Audacity: A Digital Audio Editor
+
+  ProjectFileIORegistry.cpp
+
+  Paul Licameli
+
+**********************************************************************/
+
+#include "ProjectFileIORegistry.h"
+
+#include "audacity/Types.h"
+#include <unordered_map>
+#include <wx/string.h>
+
+
+namespace ProjectFileIORegistry {
+
+namespace {
+   using TagTable = std::unordered_map< wxString, TagHandlerFactory >;
+   static TagTable &sTagTable()
+   {
+      static TagTable theTable;
+      return theTable;
+   }
+}
+
+Entry::Entry(
+   const wxString &tag, const TagHandlerFactory &factory )
+{
+   sTagTable()[ tag ] = factory;
+}
+
+TagHandlerFactory Lookup( const wxString &tag )
+{
+   const auto &table = sTagTable();
+   auto iter = table.find( tag );
+   if ( iter == table.end() )
+      return {};
+   return iter->second;
+}
+
+}