[Media Controller] Multiple artists support

According to the spec, xesam:artist should be a string array, but
majority of players uses string instead. This patch adds support for
both string array and string options to allow new software development
according to the spec but also maintain compatibility with existing
solutions.

BUG: 405762
FIXED-IN: 5.17.1

Differential Revision: https://phabricator.kde.org/D24740