The old struct config_param remains only for top-level string options.

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Allow database plugins to announce that they have been modified.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

Replaces GLib's GError.

Allow the plugin to allocate the GetSong() return value.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Remove GLib dependency from some headers.