Adding the ability to upload to OSM.

Adding the ability to upload to OSM.

A new menu entry allows to upload the data of a TrwLayer to OpenStreetMap
as a GPX data file.

Technics:
The button opens a dialog to request the needed parameters:
- authentification informations (email and password)
- a filename (initially built with the name of the TrwLayer)
- the descriptios, tags and visibility of the trace
Then, a background thread is activated.

TODO:
- Check the parameter (not empty, syntactically correct tags)
- Error handling: as the upload is threaded, the error is not displayed
- Allow to upload a simple track (not a whole TrwLayer)