Add a line item reference relationship and use it in the cart block

Currently, the shopping cart block is built using a line item View and accepts as an Argument the line item ID (or IDs). This should be changed to an Order View that accepts the Order ID as the Argument, establishes a relationship to line items via the Order's line item reference field, and then outputs the same data. There is currently no line item reference relationship so it will need to be added... but this will be huge. : D

View the product reference Views integration for an example... and I suppose we'll use this same thing for the payment reference / tab.