Cart life cycle

[MatteoPiovanelli-Laser]

part of the things discussed in #117

In this PR I am adding cart life cycle events. I made an event interface for the events on a cart I could think of:

• adding an item
• removing an item
• adding/removing in bulk
• "finalization" of the cart, meaning the customer bought what was in it.
• update, generic, called by all the others except Finalized.

I provided a default implementation for the event handler, that will trigger wrokflow activities specific for all steps.

I also made a few changes to the tokens for the cart (and added one for the item added/removed), because it wass not outputing the strings for products as I expected them to (attributes where not written out, instead having their class name as a string).

[MatteoPiovanelli-Laser]

Updated the PR

[bleroy]

Thanks!