Allow for more precise values to be sent to the shell to fix a bug
where the volume icon in the shells OSD is not the "muted" icon while
the volume bar shows a value of 0.

This happened because casting the volume to an integer value might
return 0 while the value actually is near 0. Since we also generate the
icon name inside media-keys and use the non-casted value for that, the
icon and the volume we show in the shell would be different.