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/task/task.h

@@ -30,23 +30,50 @@ class TaskDialog : public ProgressDialog
 {
   Q_OBJECT
 public:
+  /**
+   * @brief TaskDialog Constructor
+   *
+   * Creates a TaskDialog. The TaskDialog takes ownership of the Task and will destroy it on close.
+   * Connect to the Task::Succeeded() if you want to retrieve information from the task before it
+   * gets destroyed.
+   */
   TaskDialog(Task *task, const QString &title, QWidget* parent = nullptr);
 
+  /**
+   * @brief Set whether TaskDialog should destroy itself (and the task) when it's closed
+   *
+   * This is TRUE by default.
+   */
+  void SetDestroyOnClose(bool e)
+  {
+    destroy_on_close_ = e;
+  }
+
+  /**
+   * @brief Returns this dialog's task
+   */
+  Task* GetTask() const
+  {
+    return task_;
+  }
+
 protected:
   virtual void showEvent(QShowEvent* e) override;
 
   virtual void closeEvent(QCloseEvent* e) override;
 
+signals:
+  void TaskSucceeded(Task* task);
+
+  void TaskFailed(Task* task);
+
 private:
   Task* task_;
 
-  QThread* thread_;
-
-  bool task_failed_;
-  QString task_error_;
+  bool destroy_on_close_;
 
 private slots:
-  void TaskFailed(const QString& s);
+  void TaskFinished();
 
 };