shell-helper: Use doubles for shell OSD dbus calls
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.
Depends on: gnome-shell!385 (merged)