Embed title and description in GPX file

Currently, we only take the title and description from the GPX file as a suggestion for the initial title on a track upload. However, we never update the embedded title when the user provides a different one. While this doesn't matter as much on the website, it might get confusing when downloading the GPX files and viewing them in a different GPX viewer, or possibly already when viewing them with GPXViewer on fietsboek.

As such, we should update the title and description in the GPX file when the user sets a different one on fietsboek.