*: calendar, iap_mail, im_livechat, mail. Until now, one had to subscribe to the global "notification" event of the bus service and to loop through each notification to handle the one that was targeted. This PR introduces the `subscribe` method on the bus service in order to subscribe to a single notification type. closes odoo/odoo#120893 Related: odoo/enterprise#40848 Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>