Update of classes/cart.php and other code

[30knees 116]

I am looking for someone to help fix this problem:

The fix is to use the relevant code in the old PS classes/cart.php in the new classes/cart.php, see below post. However, that will also require to find all the usages of this particular cart code. Every code that calls Cart.php in order to determine price needs to be re-evaluated and potentially fixed.

Ideally, this would be done under the guidance of @datakickand @Smileand flow into the core.