Rename DocsWidget to RecentDocsWidget (as widgets for most used docs,
docs related to the currently open documents, etc. may be added in the
future), and change the title so that it doesn't abbreviate 'Docs' (for
consistency with the overlay).
Add a GConf key for favorites, and API for retrieving them.
Also add shell_app_system_lookup_basename, which we use from
the app monitor to look up WM_CLASS ids.