Ref T301, application contexts pings (by re-registering) identifiers again. By that it can be detected if the GUI is crashed

* ping timer in proxy
* utility function in timestamp list
* adjusted UI component displaying identifiers in core

src/blackcore/context/contextapplicationimpl.h

@@ -80,6 +80,9 @@ namespace BlackCore
 
         private:
             BlackMisc::CIdentifierList m_registeredApplications;
+
+            //! Housekeeping
+            void cleanupRegisteredApplications();
         };
     } // namespace
 } // namespace