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/export/exportvideotab.cpp

@@ -28,8 +28,8 @@
 
 #include "core.h"
 #include "exportadvancedvideodialog.h"
-#include "render/backend/exportparams.h"
 #include "render/colormanager.h"
+#include "task/export/exportparams.h"
 
 OLIVE_NAMESPACE_ENTER