This commit adds a PlaylistPlugin attribute "as_folder" which for now
is only enabled in the "CUE" playlist plugin (which handles separate
"*.cue" files). If a playlist with this flag set is being scanned
during database update, it will be parsed and its contents will be
added to the database. This allows clients to inspect them like
directories and its contents will be searchable.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/39
This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().
The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.