Port the payment form field

Depends on #2047919: Add a payment add form controller, and #1896416: Use contexts as payment bundles so the context can store an entity reference to the entity the form is attached to.

Field types are now plugins. See https://drupal.org/node/2064123.

This patch keeps the field in a separate module, because a configurable entity reference field is use to link payments to the entities they were made from. We may be able to make this dependency optional and merge the field into the main module anyway.

One problem is that every field requires one payment currency and an undefined number of line items. Field storage cannot handle this. Two possible solutions are:

1. Set the payment currency code in the instance, but that means that all payment forms of the same instance will have a predefined currency.
2. Find the payment currency based on the line items, e.g. use the first line item's currency.

The field type and widget plugins work and are tested. The formatter and payment context plugins need work, and so will the entity form controller that is added for the formatter to use.

Let's fix #2089375: Remove payment method plugin payment operations and #2089355: Store payments' payment method configuration before we continue with this patch, so we won't have to rework form execution later on anymore.

The three field plugins are done and tested. The payment type plugin seems to work as well, but needs more testing.

The biggest problem now is that payment type derivatives contain a colon, which is not allowed in filenames. See #2035345: Reconsider whether to use ':' as separator for derivative plugins.

Working on the upgrade path.

Because of #2035345: Reconsider whether to use ':' as separator for derivative plugins and #2107713: Remove the upgrade path, the upgrade path and entity references to the source entities were dropped.