Ref T301, core ping fix https://swift-project.slack.com/archives/G7GD2UP9C/p1534677752000100

Identifier list was filled and more and more identifiers got checked via DBus

- identifier set if only one unique identifer is needed
- using set avoids duplicates and fixes the bug
- restart time if registered, no need to check again

src/blackmisc/identifierset.h

@@ -0,0 +1,52 @@
+/* Copyright (C) 2018
+ * swift project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution and at http://www.swift-project.org/license.html. No part of swift project,
+ * including this file, may be copied, modified, propagated, or distributed except according to the terms
+ * contained in the LICENSE file.
+ */
+
+//! \file
+
+#ifndef BLACKMISC_IDENTIFIERSET_H
+#define BLACKMISC_IDENTIFIERSET_H
+
+#include "blackmisc/identifier.h"
+#include "blackmisc/collection.h"
+#include "blackmisc/variant.h"
+#include "blackmisc/blackmiscexport.h"
+#include <QMetaType>
+
+namespace BlackMisc
+{
+    /*!
+     * Value object encapsulating a set of object identifiers
+     */
+    class BLACKMISC_EXPORT CIdentifierSet : public CCollection<CIdentifier>
+    {
+    public:
+        //! Default constructor.
+        CIdentifierSet();
+
+        //! Construct from a base class object.
+        CIdentifierSet(const CCollection<CIdentifier> &other);
+
+        //! This list contains an identifier which is not contained in other.
+        bool containsAnyNotIn(const CIdentifierSet &other) const;
+
+        //! Get a list of identifiers reduced to maximum one per machine.
+        //! If there is more than one per machine, it is undefined which one will be added.
+        CIdentifierSet getMachinesUnique() const;
+
+        //! Get machine names
+        QStringList getMachineNames(bool unique = true, bool sort = true) const;
+    };
+} //namespace
+
+Q_DECLARE_METATYPE(BlackMisc::CIdentifierSet)
+Q_DECLARE_METATYPE(BlackMisc::CCollection<BlackMisc::CIdentifier>)
+// in list: Q_DECLARE_METATYPE(BlackMisc::CSequence<BlackMisc::CIdentifier>)
+
+
+#endif //guard