Added (sorta-of) screenshot support via DBus.

You can now use the DBus methods /org/semplicelinux/vera/Screenshot/{Full,CurrentWindow} to take a screenshot.
It's still pretty rough (no file save dialog, it will be saved in /tmp/prova.png) and CurrentWindow() will not
take the screenshot of the window decoration.
Added vera xsession .desktop file.
Screenshot: added option to disable the screenshooter.

The new enable-screenshot option in org.semplicelinux.vera permits
to control the startup of the internal screenshooter.