db/update: scan CUE playlist contents

This commit adds a PlaylistPlugin attribute "as_folder" which for now
is only enabled in the "CUE" playlist plugin (which handles separate
"*.cue" files).  If a playlist with this flag set is being scanned
during database update, it will be parsed and its contents will be
added to the database.  This allows clients to inspect them like
directories and its contents will be searchable.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/39

src/db/plugins/simple/DirectorySave.cxx

@@ -50,6 +50,9 @@ DeviceToTypeString(unsigned device) noexcept
 	case DEVICE_CONTAINER:
 		return "container";
 
+	case DEVICE_PLAYLIST:
+		return "playlist";
+
 	default:
 		return nullptr;
 	}
@@ -63,6 +66,8 @@ ParseTypeString(const char *type) noexcept
 		return DEVICE_INARCHIVE;
 	else if (StringIsEqual(type, "container"))
 		return DEVICE_CONTAINER;
+	else if (StringIsEqual(type, "playlist"))
+		return DEVICE_PLAYLIST;
 	else
 		return 0;
 }