[RFC] Improved object modeling

[sandermangel]

To facilitate easier integration with a wide range of backends the data models that are at the basis of what we build should be generic. Entities such as products, categories, customers and orders should be simplified and stripped of any logic tied directly to Magento. At the basis is an agnostic model which can be extended, or used, by another model specifically handling Magento or Shopware or another platforms logic.

For this, we'll create Domain Objects describing in generic, unambiguous terms what properties an object can have and what basic behaviours it should offer, from there we expand for each integration that is done the specifics for the underlying backend.