Allow symlinks outside of mp3 folder

Allow symlinks outside the specified mp3 folders. If a symlink points to a directory that hasn't been scanned yet, scan it.
Because a directory can now be scanned (via a symlink) before "itself", we now check if a fid has already been given to that directory. If so, simply add a fid to that directory instead of rescanning it.
Added support for .ignoreaschild hidden

Allows to tag a single playlist (directory) with ignore as child tag.
Just touch a .ignoreaschild file in the playlist you want to tag as "Ignore as Child"

This way, there's no need to enter the name of the folders to be ignored at runtime.