Use GDK to detect caps lock state

Close #16 (https://github.com/mate-desktop/mate-screensaver/issues/16)

Code adapted from:
https://git.gnome.org/browse/gnome-screensaver/commit/?id=d30694d542ebe7db6d254bdfd8c9ba77e4b54df2
https://git.gnome.org/browse/gnome-screensaver/commit/?id=372b4d024b367d2fa6a868a272fd994745f92fac