Updates to emms-mpris: please test!

I have pushed a new version of emms-mpris.el to the mpris branch.

What's new:
* Support changing LoopStatus and Shuffle properties
* Fix a couple of problems with the artUrl field of the Metadata property.

Many mpris controllers do not support LoopStatus and Shuffle (playerctl is
an honourable exception) but if you have access to a controller that does
support this, it would be great to have some testing!

The only thing in the mpris spec that is left for implementation is
changing the volume.  The issue here is that every volume backend reports
the volume in a different way, sigh.  So there would have to be some
unifying work on emms-volume-* first.  Is there any appetite for this?

Thanks,

---Fran