Move exo_gtk_dialog_get_action_area() to libxfceui and remove exo_gtk_dialog_add_secondary_button()

I just looked up if there is any other G_GNUC_BEGIN_IGNORE_DEPRECATIONS hidden, and there is one: exo_gtk_dialog_get_action_area() is a wrapper to turn off the deprecation warning.

GtkWidget *
exo_gtk_dialog_get_action_area (GtkDialog *dialog)
{
G_GNUC_BEGIN_IGNORE_DEPRECATIONS
    return gtk_dialog_get_action_area (dialog);
G_GNUC_END_IGNORE_DEPRECATIONS
}

thunar_column_editor_init() and exo_gtk_dialog_add_secondary_button() use this one, but the latter isn't used anywhere within Xfce project. So the only part that use this function is Thunar. However, glade_xfce_titled_dialog_post_create() in libxfce4ui has the same implementation of this (kind of) so it would be better to move this into libxfce4ui.