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/panel/taskmanager/taskmanager.cpp

@@ -35,6 +35,8 @@ TaskManagerPanel::TaskManagerPanel(QWidget* parent) :
 
   // Connect task view to the task manager
   connect(TaskManager::instance(), &TaskManager::TaskAdded, view_, &TaskView::AddTask);
+  connect(TaskManager::instance(), &TaskManager::TaskRemoved, view_, &TaskView::RemoveTask);
+  connect(TaskManager::instance(), &TaskManager::TaskFailed, view_, &TaskView::TaskFailed);
 
   // Set strings
   Retranslate();