Replace xscreensaver with a more attractive and integrated screen locker

[g7]

xscreensaver has served us well, but it's time to replace it with a more attractive solution.
Writing a screen locker is not a very simple task - and if we do one, we'll likely miss the Semplice 7 deadline.

Many distributions in the *buntu world have switched to light-locker and I guess it's the way to take.

light-locker integrates seamlessly with the current LightDM greeter, and that's a really positive thing.

Eugenio