Support for Dropin (patch)

Motivation

Adyen introduced Web Components and Dropin as a substitute for HPP redirects. Components (payment methods) are rendered on site.
This creates a cleaner interface. This patch renders the components at the payment pane.
An API-key and a merchant account need to be filled in to the configuration form.
More on Dropin
This is a start of this implementation. Still much work needs to be done.
Looking for a co-worker who is also working on this module. Let me know!

Remaining tasks

• The handling of more than the 'Authorized' result code needs to be implemented.
• Not all payment methods are implemented. Only credit card and iDeal.
• Many more work still to be done. But this is a start.

User interface changes

On the payment pane it is now possible to show all payment methods.