This returns the location in the filesystem where
files pertaining to the element should be stored. Leaves return the parent
folder, nodes return their own path. Note that this can be different than
getPath(), especially if the id3 is used to describe the layout.
Also note that the user should be able to specify that all files (art etc.) 
should NOT be stored in the filesystem, but in the data directory.

*add track->setMeta($array,$mode)
Use $this->getFileName() and set from there. Will make it very
easy for other backends to share code.

*node::getTopSubNodes()
This needs to be done.

*getRandomArt() needs to be done or removed.
