Unable to control Volume using Mpris

Hello,

I see that the mpris `Volume` properties is marked as read/write, however, it doesn't appear to be possible to change it using `playerctl -p Plexamp volume 0.6` or other dbus commands like `gdbus call --session --dest org.mpris.MediaPlayer2.Plexamp --object-path /org/mpris/MediaPlayer2 --method org.freedesktop.DBus.Properties.Set org.mpris.MediaPlayer2.Player Volume "<0.6>"`

Another thing I noticed is that when the volume slide is changed in the app, the volume level does change, however, the mpris `Volume` value remains. However, if I skip to the next song, the mpris Volume value then gets updated
```
user@machine:~$ playerctl -p Plexamp volume
1.000000
####(change volume in app here)
user@machine:~$ playerctl -p Plexamp volume
1.000000
user@machine:~$ playerctl -p Plexamp next
user@machine:~$ playerctl -p Plexamp volume
0.570000
user@machine:~$ playerctl -p Plexamp volume 0.3
user@machine:~$ playerctl -p Plexamp volume
0.570000
```

would it be possible to update Plexamp so:
1. the mpris volume updates when the slider is moved (currently, the value only updated on track change or play/pause)
2. the app volume can be updated using mpris (this would allow to change the volume from the terminal or when using widgets)