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/dialog/progress/progress.h

@@ -34,12 +34,20 @@ class ProgressDialog : public QDialog
 public:
   ProgressDialog(const QString &message, const QString &title, QWidget* parent = nullptr);
 
+protected:
+  virtual void showEvent(QShowEvent* e) override;
+
+  virtual void closeEvent(QCloseEvent *) override;
+
 public slots:
   void SetProgress(int value);
 
 signals:
   void Cancelled();
 
+protected:
+  void ShowErrorMessage(const QString& title, const QString& message);
+
 private:
   QProgressBar* bar_;