Fix gtk_widget_get_root_window deprecation

The function gtk_widget_get_root_window has been depcreated in GTK+ 3.12
in favor of gdk_screen_get_root_window. This gets rid of the deprecation
warning.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge pull request #35 from nkoep/gtk-widget-get-root-window-deprecation

Fix gtk_widget_get_root_window deprecation