#28 Nspire nested folders support

[Lionel Debroux]

Members of the Nspire family, running OS 2.x and later, support more than one level of folders; libti*/tilp does not, although it does no longer crash when meeting sub-folders.

At a minimum, dirlist needs to be rewritten, in order to traverse the list of folders and files recursively, and also to support files which have no folder name. But nested folders support can require changes at other places: for instance, when retrieving the filename in a path containing multiple '/', the '/' that matters is the last one.