Store qStartStop state in atomic for background thread

Closes #4882.

rtgui/batchqueuepanel.h

@@ -19,6 +19,8 @@
 #ifndef _BATCHQUEUEPANEL_
 #define _BATCHQUEUEPANEL_
 
+#include <atomic>
+
 #include <gtkmm.h>
 #include "batchqueue.h"
 #include "saveformatpanel.h"
@@ -53,6 +55,8 @@ class BatchQueuePanel : public Gtk::VBox,
 
     IdleRegister idle_register;
 
+    std::atomic<bool> qStartStopState;
+
 public:
     explicit BatchQueuePanel (FileCatalog* aFileCatalog);
     ~BatchQueuePanel();