Fix some issues with GTK+ 3.8.

Signed-off-by: Jean-Philippe Fleury <contact@jpfleury.net>
Fix menubar background, it should be gradient.

Signed-off-by: Jean-Philippe Fleury <contact@jpfleury.net>
Set proper padding for the menu separators.

Signed-off-by: Jean-Philippe Fleury <contact@jpfleury.net>
Un peu de ménage