"Order paid" event is not triggered correctly for free orders

Problem/Motivation

An "order paid in full" event has been added in 2.10 (see #2856586: Add an "order paid" event).
That event is triggered from the order doPreSave method and rely on isPaid() to check triggering. However, this method itself relies only on checking order balance, which lead "order paid in full" event to be triggered as soon as the order balance goes to 0, which not be when the customer has actually paid or even ordered.

Reproduction step

You will need a promotion of the type "100% off total order" and a coupon, let's call it "FREE" for that promotion.
You will also need the "coupon redemption" pane to be added on your order process: lateral bar is by default.
- Add a product to cart worth non 0 dollars
- Validate your cart and enter the order process
- When available enter the FREE coupon: the order balance now goes to 0$
The order paid in full event is triggered here but should not
- Change your mind, remove the coupon, whatever. From here, the event has been sent, maybe the customer received a receipt or whatever process was attached to an order he did not actually made at that point.

Remaining tasks

Proposal 1: Change the isPaid() method to also check total_paid is not zero
Cons: We cannot rely on this event for anything that has to be done for each order - for instance, a confirmation email - because it won't get sent for order with no payment associated (either no payment module activated, or FREE coupon for instance)

User interface changes

None

API changes

Probably not !

Data model changes

None