Register factory functions with DirManager to interpret XML tags...

... Removes direct dependency on ODManager.h and some BlockFile subclasses,
but does not yet break any dependency cycles

src/blockfile/ODDecodeBlockFile.cpp

@@ -29,6 +29,7 @@ The summary is eventually computed and written to a file in a background thread.
 
 #include "../DirManager.h"
 #include "../FileFormats.h"
+#include "../ondemand/ODManager.h"
 #include "NotYetAvailableException.h"
 
 const int bheaderTagLen = 20;
@@ -524,3 +525,11 @@ void ODDecodeBlockFile::ChangeAudioFile(wxFileNameWrapper &&newAudioFile)
 
 
 
+static DirManager::RegisteredBlockFileDeserializer sRegistration {
+   "oddecodeblockfile",
+   []( DirManager &dm, const wxChar **attrs ){
+      auto result = ODDecodeBlockFile::BuildFromXML( dm, attrs );
+      ODManager::MarkLoadedODFlag();
+      return result;
+   }
+};