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/task/CMakeLists.txt

@@ -16,12 +16,14 @@
 
 add_subdirectory(cache)
 add_subdirectory(conform)
+add_subdirectory(export)
+add_subdirectory(project)
 add_subdirectory(proxy)
+add_subdirectory(render)
 
 set(OLIVE_SOURCES
   ${OLIVE_SOURCES}
   task/task.h
-  task/task.cpp
   task/taskmanager.h
   task/taskmanager.cpp
   PARENT_SCOPE