It was only used in filetree.c. It's still implemented in Lua so
scripts using rb.playlist_add() won't break, but has been removed
from the Lua API "backend".
Change-Id: I5625a47f0692456008c6b10dee14755151d22f29
The way to call the playlist functions has changed
rb.playlist("option", var)
rb.playlist_add(filename) = becomes rb.playlist("add", filename)
added playlist.lua to the includes for conversion to old functions
if your script is broken by this change you simply add `require("playlist")`
to the top for the old functionality
added rb.playlist_tracks(dir, filename) to playlist.lua
this will allow you to add all tracks in a playlist.m3u8
to a lua table
Change-Id: I87fcc56be365d8495d214f069331b6ddbfbef1db