Enable individual track to GPX export via the Track sublayer menu.

Enable individual track to GPX export via the Track sublayer menu.

Internally gpx.[ch] supports writing a track to the specified file, so make this available in the GUI.
Expand export definitions in file.h, use the gpx.h interface as necessary routing via a_file_export function.
Filename used is automatically generated from the track name with a '.gpx' appended if appropriate.
Restructure trw_layer_export function to support passing track information.
Add the new sublayer menu entry to use this feature.