BugFix Safe access to xmldoc in snapshots and undo history

Added mutex to Action manager for access to undo history and added mutex to session's snapshots. Lock and unlock those mutex for all write access and long read access.

src/Session.h

@@ -2,7 +2,6 @@
 #define SESSION_H
 
 #include <mutex>
-#include <variant>
 
 #include "SourceList.h"
 #include "RenderView.h"
@@ -34,6 +33,7 @@ struct SessionSnapshots {
 
     tinyxml2::XMLDocument *xmlDoc_;
     std::list<uint64_t> keys_;
+    std::mutex access_;
 };
 
 class Session