bin/disorder-notify: Maybe report currently playing track on state change.

bin/disorder-notify: Maybe report currently playing track on state change.

This involves fiddling with the `now_playing' function so that it can
find out the current track for itself if necessary.

(Initially I did this on `enable_random' and `enable_play', but that
leads to duplicate now-playing reports because a `playing' notification
will follow unless we never actually stopped playing anything anwyay.
So now we only report on resume.)