use pwx for Trainingstagebuch upload

as current tcx writer currently doesn't support laps (and tcx doesn't
allow non-linear/overlapping intervals, at all) this patch makes GC upload
exercises as pwx.

This should be applied after the PWX fixes I put onto a seperate branch for clarity (there is no diff/patch dependency).

This is agreed with Niels (who owns Trainingstagebuch.org)