Add optional download dir preference

User can specify some directory in their public downloads directory where we'll place downloaded torrents in. An empty preference indicates to leave the torrents in the default download location.

Example: User specifies "what", we'll place torrents in the external storage downloads directory under what/. The notification will also be updated with this moved file URI so it will still open properly through the notification as well.
Merge branch 'feature-user-download-pref' into develop