Set the volume relatively

Property definitions

mpd $ MPDConnection :: relative_volume=
	# Set the volume relatively
	fun relative_volume=(diff: Int)
	do
		if socket == null then connect

		var status = status
		if status != null then
			var vol = status.volume
			if vol != null then
				var new_vol = vol + diff
				new_vol = new_vol.clamp(0, 100)
				volume = new_vol
				return
			end
		end

		error = "Cannot get volume"
	end
lib/mpd/mpd.nit:123,2--140,4