Update Downloader to 3.0.2

Fixes most of the bugs that were affecting deviantART downloads and
improves general stability and usability.

Changes:
- HttpClient exception handling cleaned up. Now downloads are less
likely to break an entire slot when they hit server-side problems.
- New status interface. The status column's functionality has been
replaced by a series of smaller columns indicating an item's progress
using symbols. The status column itself is now used to track error
codes.
- META column removed for now.
- New status tracking API. Now uses int status/error codes instead of
string literals.
- Add interface revamped and less ugly! Includes functionality for
adding items from file.

Known issues:
- Request sending sometimes doesn't work with deviantART items.
- Overall progress tracker sometimes tracks fewer items completed than
expected (esp. with deviantART).
Add //TODO items to SettingsPane (minor change)