Fix the sort order of track listing.

As mentionned by many users, new tracks were listed in no particular order, basicaly relying on the OS to do the sorting. Now, it sorts the files based on their modified timestamp.


See these forums posts for reference:
http://devfarm.it/forums/topic/track-listing-newest-first/#post-24404
http://devfarm.it/forums/topic/sorting-gpx-file-listing/
https://wordpress.org/support/topic/sort-folder-not-working-files-appearing-in-no-particular-order-patch-included