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.cpp

@@ -61,8 +61,12 @@ TaskViewItem::TaskViewItem(Task* task, QWidget *parent) :
 
   // Connect to the task
   connect(task_, &Task::ProgressChanged, progress_bar_, &QProgressBar::setValue);
-  connect(task_, &Task::Removed, this, &TaskViewItem::deleteLater);
   connect(cancel_btn_, &QPushButton::clicked, task_, &Task::Cancel, Qt::DirectConnection);
 }
 
+void TaskViewItem::Failed()
+{
+  task_status_lbl_->setText(task_->GetError());
+}
+
 OLIVE_NAMESPACE_EXIT