MythMusic: fix the notracks text not always showing/hiding when it should

This fixes a problem with the "You haven't selected any tracks to play" text
not updating properly when adding or removing single tracks from the active
playlist.

Also work around a bug in MythUIButtonlist not removing the last item properly.
(cherry picked from commit 088d684558a3197b022695e39b3c50d89bca92f7)