[13.x] Fix receipt totals and balance

[driesvints]

This PR fixes receipt totals when a balance is applied.

Here's an example with the current Cashier receipt:

And here's the Stripe invoice:

As you can see these numbers do not match. There's no reason the total customer balance should be displayed and the final total invoice amount isn't correct (should be €12.10).

With this PR it gets fixed to:

So it's the same as the Stripe one. I've added new methods to the Invoice object to accommodate for these changes.

There's one note I'd like to make: the current behavior of the Invoice's total method is actually incorrect. I've added a new realTotal method to temporarily accommodate for the true value. I did not want to break backwards compatibility which is why I opted for a new method. In Cashier v14, I strongly want to urge to change the behavior of the total method and remove the realTotal method again.