Migrating payment extensions to OpenCart 2.2 and 2.3

This is an incomplete list of changes.

In OpenCart 2.2.x

• In catalog/model/ there is no function currency->getCode()
  (Fatal error: Call to undefined method Cart\Currency::getCode() in ...on line ...)
  //$currency = $this->currency->getCode(); // OC <= 2.1
  $currency = $this->config->get('config_currency'); // OC 2.2
• In controllers, the prefix to default templates has been automatically added.
  Remove the "default/template" from load->view()

In OpenCart 2.3.x
Along with the listed changes for OpenCart 2.2.x we have noticed these changes:

• Directory structure has been changed, from catalog/controller/payment/ to catalog/controller/extension/payment. This affects view files, breadcrumbs in the admin as well as the controller class name, files and URL routes.