Make the player status a per-player object in the MPRIS plugin

Summary:
This directly fixes a couple of bugs (where the UI was not updated in all cases) and will allow different code parts to use different players without clashing with each other.

This is required for sensible behaviour while using the media control notification (which I plan to work on).

Reviewers: KDE Connect, thomasp, albertvaka

Reviewed By: KDE Connect, thomasp, albertvaka

Subscribers: albertvaka, apol, thomasp

Differential Revision: https://phabricator.kde.org/D8942