Adapt to avoid use of deprecated GTK3 style & size APIs
    
    The GtkStyle API has been deprecated in favour of GtkStyleContext.
    Update ovBox.c to use the latter if building with GtK3. Also replace
    use of the gtk_widget_size_request API with gtk_widget_get_preferred_size.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>