add SaveToJsonFile() function from SM4 betas.

src/Song.cpp

@@ -1049,6 +1049,12 @@ bool Song::SaveToSSCFile( RString sPath, bool bSavingCache )
 	return true;
 }
 
+bool Song::SaveToJsonFile( RString sPath)
+{
+	LOG->Trace( "Song::SaveToJsonFile('%s')", sPath.c_str() );
+	return NotesWriterJson::WriteSong(sPath, *this, true);
+}
+
 bool Song::SaveToCacheFile()
 {
 	SONGINDEX->AddCacheIndex(m_sSongDir, GetHashForDirectory(m_sSongDir));

src/Song.h

@@ -116,6 +116,10 @@ public:
 	bool SaveToSSCFile( RString sPath, bool bSavingCache );
 	/** @brief Save to the SSC and SM files no matter what. */
 	void Save();
+	/** 
+	  * @brief Save the current Song to a JSON file.
+	  * @return its success or failure. */
+	bool SaveToJsonFile();
 	/** 
 	  * @brief Save the current Song to a cache file using the preferred format.
 	  * @return its success or failure. */