Fix a memory leak in Song (dynamically allocated error list)

Make the member Song::m_errors a static member to prevent memory leaks.
The member was allocated dynamically when an instance of a Song was
created but was not deleted in the destructor.

include/Song.h

@@ -367,7 +367,7 @@ private:
 
 	bool m_loadingProject;
 
-	QList<QString> * m_errors;
+	QList<QString> m_errors;
 
 	PlayModes m_playMode;
 	PlayPos m_playPos[Mode_Count];