PaymentRequestUI should have methods editSection(SectionType, EditorDescription) and addToSection(SectionType, EditorDescription).

The EditorDescription class should contain a double-array of fields to be arranged on the page:

[[      field 0       ]
,[      field 1       ]
,[ field 2 , field 3 ]]

Each field should be either: a text field, a drop down, or a checkbox.

A text field always has:
 - id
 - label

A text field may also have:
 - Pre-filled value
 - autocompletion candidates
 - helper icons (e.g. [visa][mc][amex])
 - floating action icon [scan card number]
 - "required" flag
 - "multi-line" flag
 - validation callback


|---------------|------------------|
| [helper icon] | [helper icon]    |
|---------------|------------------|
|           [floating action icon] |
| ________________________________ |
|----------------------------------|


A drop down should have a list of id-label pairs. Changing the drop-down should immediately notify the PaymentRequestUI.Client, which may update the fields.

|----------------------------------|
| Default option label           v |
|----------------------------------|


A checkbox should have a label, possibly with a link.

|-----|---------------------------|
| [ ] | This is some text that    |
|     | can break into multiple   |
|     | lines. This text can have |
|     | spannable HTML links.     |
|-----|---------------------------|

When user clicks [SAVE] button, PaymentRequestUI should send the state of the editor to PaymentRequestUI.Client.