cellMusic: fix some custom music issues

[Megamouse]

• Fix volume of custom music (this was always either on 0% or 100% due to a wrong volume range passed to the player)
• Mark playlists as valid as soon as the playlist is set (this was previously only done after loading them from file)
• Always provide the whole playlist when a track is selected in an overlay dialog (we previously only ever returned the track as a single song playlist)
• Silence some cellRtc log spam

This fixes the music player in LBP3. I hope this doesn't break any other games.
NOTE: rewind doesn't work because Qt doesn't support it. So don't complain about that xD

fixes #16649