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/contextapplication.h

@@ -212,6 +212,8 @@ namespace BlackCore
             virtual QString dotCommandsHtmlHelp() const = 0;
 
         protected:
+            static constexpr int PingIdentifiersMs = 20000; //!< how often identifiers are pinged
+
             //! Constructor
             IContextApplication(CCoreFacadeConfig::ContextMode mode, CCoreFacade *runtime);