Ref T171, use a shared completer for callsign completion

* CSharedStringListCompleter utility class supporting timestamp checks
* avoid unnecessary DBus transfer
* less memory consumption

Remark: No totally clear if a QCompleter can be shared, but I have not noticed a problem so far

src/blackgui/sharedstringlistcompleter.h

@@ -0,0 +1,54 @@
+/* Copyright (C) 2017
+ * 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 BLACKGUI_SHAREDSTRINGLISTCOMPLETER_H
+#define BLACKGUI_SHAREDSTRINGLISTCOMPLETER_H
+
+#include "blackgui/blackguiexport.h"
+
+#include <QCompleter>
+#include <QStringList>
+#include <QScopedPointer>
+
+namespace BlackGui
+{
+    /*!
+     * Completer shared among multiple UI elements.
+     * Normally used as static member
+     */
+    class BLACKGUI_EXPORT CSharedStringListCompleter
+    {
+    public:
+        //! Constructor
+        CSharedStringListCompleter() {}
+
+        //! Update data
+        bool updateData(const QStringList &data, int cacheTimeMs);
+
+        //! Clear completer data
+        void clearData();
+
+        //! Was updated within check time
+        bool wasUpdatedWithinTime(int checkTimeMs) const;
+
+        //! Completer
+        QCompleter *completer() const { return m_completer.data(); }
+
+        //! Convert to completer
+        operator QCompleter *() const { return m_completer.data(); }
+
+    private:
+        qint64 m_lastUpdated = 0;
+        QScopedPointer<QCompleter> m_completer { new QCompleter(QStringList()) }; // empty list required to init model
+    };
+} // ns
+
+#endif // guard