applets/taskmanager: `xesam:artist` is a stringlist
According to https://www.freedesktop.org/wiki/Specifications/mpris-spec/metadata/#xesam:artist the data type of "xesam:artist" is List of Strings. This fixes empty artist when there are more than one artist. ![image](/uploads/1513b54a28b8adce6ea8322aee020eea/image.png)