Replace libexo with libxfce4util
Related : exo#82
- exo_str_is_equal() => g_strcmp0() == 0
- Include gettext using libxfce4util instead of libexo
- Drop libexo dependency and depend on libxfce4util>=4.0.0 instead (xfce-i18n.c was added in libxfce4util==4.0.0)