elisa: implement libmpv backend, preferred over libvlc

As discussed in https://bugs.kde.org/show_bug.cgi?id=461299, while the stutters were reduced by !397 (merged), they were still audible.

At least some of the ones i have heard, and looked at, appeared to be artifacts of the source audio, but Aurélien COUDERC noted, that those stutters weren't limited to me.

As discussed in a follow-up, at least some of the stutters appear to be only present in VLC, but not elsewhere.

Thereby, i would like to propose to move away from VLC, to MPV. Personally, i've essentially never used VLC, but MPV, and basically never had any audio issues with it. Additionally, it has much more control surfaces, should one want to go nuts with tuning.

Please note, i've just wrote this. While i have fixed all the bugs i have encountered, there are probably some more left.

I would like to acknowledge that having 3 backends is problematic, but i'm not sure if we can just drop the VLC one.