Plug priorities reconsidered

Written for Pantheon Plugs by Sergey "Shnatsel" Davidoff on 2012-03-07

Since Switchboard supports unmodified GNOME Control Center plugs (props and kudos to xapantu for implementing this!), plug priorities have been reconsidered. Plugs with decent GNOME equivalents now have low priority, plugs with unsatisfactory upstream equivalents are high priority, and plugs upstream equivalents of which are insufficient for our use case are considered essential.