Get the cursor position on the song duration.

Property definitions

mpd $ ServerStatus :: time_ratio
	# Get the cursor position on the song duration.
	fun time_ratio: nullable Float do
		if time_at == null or time_total == null then return null
		return time_at.to_f / time_total.to_f
	end
lib/mpd/mpd.nit:238,2--242,4