Implement unobstrutive desktop notifications for jabber client.

Jabberclient will now use the operating system's notification service to notify when there is a new message.

I tried to make it behave as much unobstrutive as possible :

• informative : one notification is created per conversation.
• but not distractive : if there is already one notification open, the next message in the same conversion won't create a new notification.
• with no hassle : notifications are deleted when jabberclient is focused.

@nexedi : you can try it on my dev instance : https://softinst76104.host.vifib.net/erp5/web_site_module/jabber_client/ . Tell me what you think of the behavior of the notifications and if it should be improved. I think it is currently a good middle ground as few notifications mean I can ignore them intentionnaly (ie: I will answer later, after finishing my task at hand) or unintentionnaly (iow: I'm too busy on my task and I don't see it), but still being useful (ie: I come back from lunch and I can see 2 people jabbered me).

For information :

• the notification API is disabled in private windows and in chromeos/chromiumos/nayuos guest mode.
• the style is not mine, but it is what's currently commited in generic ERP5.

If no comment/issue I will merge it next Friday (December 11th)