Add traking on warn fields

WORK IN PROGRESS: port hr_holidays_usability

account_usability/partner.py

@@ -0,0 +1,11 @@
+# -*- coding: utf-8 -*-
+# © 2017 Akretion (Alexis de Lattre <alexis.delattre@akretion.com>)
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
+
+from odoo import models, fields
+
+
+class ResPartner(models.Model):
+    _inherit = 'res.partner'
+
+    invoice_warn = fields.Selection(track_visibility='onchange')