task: massively simplified task system

Made various changes and fixes to the task system:

- Tasks are built around QtConcurrent rather than QThread. Reduces
  code complexity significantly.
- Task error reporting is now streamlined in both TaskManager and
  TaskDialog.
- Moved ProjectImport/Save/LoadManager to the app/task folder

app/widget/taskview/taskviewitem.h

@@ -45,6 +45,8 @@ class TaskViewItem : public QFrame
 public:
   TaskViewItem(Task *task, QWidget* parent = nullptr);
 
+  void Failed();
+
 private:
   QLabel* task_name_lbl_;
   QProgressBar* progress_bar_;