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/cli/clitask/clitaskdialog.cpp

@@ -26,7 +26,6 @@ CLITaskDialog::CLITaskDialog(Task *task, QObject* parent) :
   CLIProgressDialog(task->GetTitle(), parent)
 {
   // FIXME: Still developing this, don't try to use
-  task->Start();
 }
 
 OLIVE_NAMESPACE_EXIT