Customizer: Create a Simple Payments Button from the Customizer Widget UI

[rodrigoi]

Epic: #24617
Depends on #24614

Add the ability to create a new Simple Payments Button from the Theme Customizer Widgets tool, or replace an existing widget with a new Simple Payments Button created on the Customizer.

Steps:

• Open the Theme Customizer
• Click on Widgets
• Click on Add a Widget or select an existing Simple Payments Widget.

The widget module should show a list of existing Simple Payment Buttons, with either the first item selected, or the previously selected Payment Button.

By clicking on the Add + button below the payment buttons drop down list, a form should expand with the same fields the Editor UI has, and should be integrated with the media library for image selection.

All changes should be reflected live on the Customizer Preview. Saving the Simple Payment Button effectible saves it and makes it available everywhere Payment Buttons are accessed.
When the Payment Button is saved, the drop down list should update the list, and make the new Button the selected item.
Only when clicking Publish the new selection is persisted.

Notes:

Simple Payments Sprint – Widget Design: p9Jlb4-5q-p2
GM 2017 Project: Simple Payments Widget: https://github.com/Automattic/jetpack/blob/gm-17/payment-widget/modules/widgets/simple-payments.php

[rodrigoi]

Ready for Review on Automattic/jetpack#9782.