Provide priority mechanism for sorting calls of addons

given an engine notification, some addons should be called before or after some other addons.

Currently, we have no way to sort them.

Some difficult point, we may not know about the other addons, (so we cannot just create a dependency between addon), the priority may be different on the various calls (ex: high priority on engine start, low priority on executeStep)

So I suggest a mechanism based on priorities that are set on each notification.
By default, the priority would be "0", and the higher it is, the earlier it should be run, the lower it is (negative) the later it should be run.

Then before launching a notification, it will sort the addons to be run according to these priorities directives.