Add More info to GPX export

[eMerzh]

Here is a little patch to add infos to gpx export. Rss feed export contains much more info than GPX but gpx allow to export more than 10 items.

Gpx allow more info stored in std elements

Thanks

[emka]

As discussed via mail: this patch would break existing applications. We need to come up with something else to include details or allow more items in other formats.