Image streams were initially separated from video streams, but they're now
joined with a parameter defining if they're a still image, image sequence,
or regular video. The image sequence import process has also improved so
that if images from the same sequence are imported too, they'll either be
ignored or the user won't be asked again for those if they should be an
image sequence (fixes#1193). Also shifts decoder "probe" process to return
an item, useful if the decoder returns a non-footage item.
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
itsmattkc