fix(snap): declare mpris as slot instead of plug

MPRIS was declared as a plug (control other players) but QBZ needs to
register as an MPRIS media player (be controlled by media keys). Change
to a slot with name "qbz" so snapd exposes org.mpris.MediaPlayer2.qbz
on the session bus.

snapcraft.yaml

@@ -82,7 +82,13 @@ apps:
       - pulseaudio
       - alsa
       - pipewire
-      - mpris
+    slots:
+      - mpris-player
+
+slots:
+  mpris-player:
+    interface: mpris
+    name: qbz
 
 parts:
   qbz: