mpris: Use a scope specific message instead of a global one

mpris: Use a scope specific message instead of a global one

Since the mpris implementation of the notification tray supports showing
multiple notification (one for each player), it doesn't make sense to
have only one global property to store the message, since that only
allows referencing one message at a time.

Instead, handle the MediaMessages completely inside the scope of
`_addPlayer()`, this should allow showing more than one message again,
which broke with commit 4dc44304dff2974d3b026b5aa58e0d5c25d38216 [1].

[1] https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/791

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/833