Add notifications when new torrent file/magnet link is added to the download list

Related to https://github.com/qbittorrent/qBittorrent/issues/334

It is not a very clean implementation, but I have no experience with QT, so I hope you might clean it up when applying these changes, otherwise these commits achieve two things

1. Add notifications when torrents/magnet links are added by clicking in a browser and the add torrent dialog is disabled.
2. "Fix" notifications on Linux Mint 14. 
  A check was being done, which seems to fail on my distro atleast, which led to me getting ugly qt notifications instead of the system ones, I have commented the check, since I cant find any documentation pointing towards that function or the need for it.