Update worksheets from GeoGebra

[florian]

Instead of an one-time upload like it is implemented at the moment there should be a closer bound between GeoGebra and GeoGebraTube: Users should be able to choose whether to re-upload or update their worksheet if they already uploaded it to GeoGebraTube. This requires the identifier passed to GeoGebra to be permanent and to be stored inside the GeoGebra worksheet. As no GeoGebraTube-specific authentication will be implemented in GeoGebra the update-process always requires the user to open a browser window and at least confirm the update.