media-players: New lockscreen plugin to track all mpris players

One often has multiple mpris capable players running (e.g. podcasts and music player). The lock screen plugins allows to switch between them without effort.

We first extend mpris manager to keep track of running players, then free media-player from being tied to the default player and finally add the plugin.

Marking as draft as I've mostly written it down and didn't look back so handle with care. We should likely disable "can-raise" as that makes no sense on the lock screen.