taskmanager: ensure task is finished before deleting it

Fixes race condition that could result in a segfault if a Task hadn't emitted
Finished() before it was destroyed by TaskManager.

app/task/taskmanager.h

@@ -142,8 +142,6 @@ private:
    */
   void DeleteTask(Task* t);
 
-  void TaskFinished(Task *task);
-
   TaskStatus GetTaskStatus(Task* t);
 
   void SetTaskStatus(Task* t, TaskStatus status);
@@ -173,6 +171,8 @@ private slots:
 
   void TaskFailed();
 
+  void TaskFinished();
+
 };
 
 #endif // TASKMANAGER_H