stop relying on inactive/deprecated gnome-screensaver project

Currently, launching a gnome-flashback session depends on gnome-screensaver, an inactive/deprecated component from GNOME. Not providing the ancient component will result in issues like #676-Unable to perform screen lock on GNOME Flashback.

The Gnome-Flashback Project page mentions this issue as a problem being targeted, but no activity tracked:

Current problems being targeted

    Visibility of Desktop icons aren't settable by dconf, see Bug 695088
    gnome-screensaver?

Last release from gnome-screensaver dates back to 3.6.1, from 6 years ago.

Hopefully gnome-flashback should implement its own screen lock mechanism or rely in a more-active project (e.g: xscreensaver).