First commit for Satellite tracks. It doesn't work yet, but the framework is reasonably complete. Bernard, let me know if you have problems adapting your code to this framework. In addition to adding Jean-Baptiste Butet's SatLib library, I added two new classes:

1) SatelliteTrack: this encapsulates a single satellite track across the sky, as a QList<SkyLine*>
2) SatelliteComponent: component for managing all the current satellite tracks.

FIXME: I am getting "undefined reference" linker errors for the two
SatLib functions I use. I can't figure out what the reason is; maybe
there's something wrong with one of the CMakeLists.txt files. Both of
the SatLib functions are in SatelliteComponent::init(). To keep trunk
working, I have commented out this function for now. I would really
appreciate it if some people could try uncommenting the function to see
if they get the linker errors as well.