Add unmoderated queue notifications

A new checkbox can be found in the toolbox settings: "Get unmoderated
queue notifications".

Disabled by default (due to potentially high notification volume), when
enabled, this addition provides desktop notifications of new items in
the unmoderated queue of the subreddits tracked by the unmoderated
counter.
Merge pull request #198 from Sabenya/unmoderated-notifications

Add option for unmoderated queue notifications