gtk_widget_override_font has been deprecated since GTK 3.16
and should not be used in newly-wrriten code. They suggest using
a CSS style class using application-specific GtkStyleProvider
and a CSS style class.

However, considering that in the case of a GtkLabel we could also
use gtk_label_set_attributes.In this patch, all deprecated
gtk_widget_override_font was replaced using GtkLabel scheme.

Change-Id: I799bd2e07168a8f9adde9057bbdef9d03a16231c
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>