Update downloader to v3.0

TESTED FOR DANBOORU, NOT FOR DEVIANTART OR GENERAL

Changes:
- Threading updated so that a single thread is maintained for each
download slot.
- A single HttpClient is used for each download slot (hopefully solving
IO exceptions).
- Added reset button (functionality to come).
- Reorganized table columns (functionality to come).

Known issues:
- broken references in deprec folders
- slow loading time for add window