Plasma::Service API addition

[Alex Merry]

So I was about to commit the new nowplaying QML plasmoid to
kdeplasma-addons, when I realised that it depends on the API additions
in https://git.reviewboard.kde.org/r/104760/, which doesn't appear to
have been looked at.

Now we're actually past the soft feature freeze, and while I put the
mpris2 dataengine and nowplaying plasmoid in the feature list, the new
API isn't in the feature list.

What's the best thing to do here?

Alex