Implements a stable key and consistent export for opencaching waypoints

the latest fix by @Bananeweizen solved the problem of the waypoint multiplication for opencaching on download from the website.
This fix adds a stable key to the oc waypoints, so they retain their customized content on refresh.
Additionally was the gpx-export modified to use the same gsak extension to link waypoints to its parent cache as opencaching uses.
To fix the import side, the gpx parser needs to be rewritten heavily, as we curently import caches and waypoints temporarily into a geocache object.