Looping sounds were always showing as 'Looping', even after they
stopped. Detection of stopped sounds is much better now.
Instead of only logging a sound as stopped when the audio source
is destroyed, we now detect whether or not it is associated with
an audio channel or not (the normal way to stop a sound is to set
the channel to NULL). This is still a bit fuzzy, since an
audio channel doesn't necessarily have to be playing, but states
where an audio source is associated with a non-playing audio channel
are only short temporary states that don't affect the usefulness
of the sound explorer.
