Added support for playlists to be written on devices and export of tracks in playlist order

I've merged my patches for the old iTunes Agent with the newest Notpod sources (see http://www.crea-doo.at/weblog/2011/01/05/itunesagent-1-3-4-patch-sync-more-than-one-folder-and-playlist-support/).
Two functionalities are added:
- Support for writing various playlists on the device (m3u, m3u extended, wpl, zpl)
- Export of tracks possible in the original playlist order (new sync pattern)

Furthermore I did a bit of clean up in the serializable classes for im-/exporting the xml settings.