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.
To avoid loading applications from two different systems, use
ShellAppSystem solely. This unifies the initial load and the
reload.
Extend ShellAppSystem to also load settings/preferences, and
ensure they appear in the search.
