aft: Update status items on afs events.

When a paraslash command changes the afs or afh info structures of the
current audio file, for example by re-adding the file with modified
meta tags, the stat output still reports the old values. With this
patch applied, the status items are updated immediately.

This is achieved by letting aft.c honor the AFSI_CHANGE and AFHI_CHANGE
events. If the audio file which triggered the event is the one
currently being streamed, the event handler recreates the status items.
We need two new global static variables in aft.c to track the row of
the audio file table which corresponds to the current file.