display shred duration after completion

include/drive.h

@@ -38,6 +38,7 @@ private:
     time_t u32Timestamp = 0U; //unix timestamp for detecting a frozen drive
     double d32TaskPercentage = 0U; //in percent for Shred (1 to 100)
     time_t u32TimestampTaskStart = 0U; //unix timestamp for duration of an action
+    time_t u32TaskDuration = 0U; //time needed to complete the task
 
 private:
     void setTimestamp();
@@ -79,6 +80,9 @@ public:
     void setActionStartTimestamp();
     time_t getActionStartTimestamp();
 
+    void calculateTaskDuration();
+    time_t getTaskDuration();
+
 };
 
 #endif  // DRIVE_H_