Use temp dir. for cache when a unit test is performed

* the temp. dir can also be used for other purposes

src/blackcore/application.h

@@ -22,6 +22,7 @@
 #include <QScopedPointer>
 #include <QString>
 #include <QStringList>
+#include <QTemporaryDir>
 #include <atomic>
 #include <functional>
 
@@ -188,6 +189,9 @@ namespace BlackCore
         //! Save all settings
         BlackMisc::CStatusMessage saveSettingsByKey(const QStringList &keys);
 
+        //! Directory for temporary files
+        QString getTemporaryDirectory() const;
+
         //! Run event loop
         static int exec();
 
@@ -434,6 +438,7 @@ namespace BlackCore
         QScopedPointer<CSetupReader>             m_setupReader;            //!< setup reader
         QScopedPointer<CWebDataServices>         m_webDataServices;        //!< web data services
         QScopedPointer<BlackMisc::CFileLogger>   m_fileLogger;             //!< file logger
+        QTemporaryDir                            m_tempDirectory;          //!< temp.directory for the lifetime of application object
         QNetworkAccessManager                    m_accessManager { this }; //!< single network access manager
         CCookieManager                           m_cookieManager;          //!< single cookie manager for our access manager
         QString                                  m_applicationName;        //!< application name