Customize: changeset Publish button and panel focus management

[afercia]

The new Customiser changeset feature introduces a publish button with a coupled "cog" button and an expandable panel:

There are 2 relevant accessibility issues here, related to focus management:

• the Publish and "cog" buttons focus style relies just on a color change and it's barely perceivable; there should be a clear indication of focus, made with a shape and not just color
• when clicking the "cog" button, the related panel expands on the right; however, the panel is placed in the source after the site preview; when using a keyboard to navigate content, this makes very difficult to even understand what is going on and requires potentially dozens of Tab key presses to actually reach the panel. As done for other panels (available menu items, available widgets) focus should be managed programmatically and moved to the expanded panel. When tabbing away from the panel or closing it, keyboard navigation should be moved back to the sidebar.

Note: Ideally, the expanded panel should be placed in the source immediately after the controls that opens it. I guess this is not possible right now, due to technical limitation of the current Customizer implementation. If so, the only option left is managing focus programmatically.

[westonruter]

(This can be fixed after Beta1 to give priority to enhancements and new features first before deadline.)

[sayedwp]

Work is in progress here ​https://github.com/xwp/wordpress-develop/pull/279

[westonruter]

In 41802:

Customize: Improve keyboard accessibility for publish settings section.

Props sayedwp.
See #39896.
Fixes #42027.

[sayedwp]

Improvements are in progress here ​https://github.com/xwp/wordpress-develop/pull/283

[westonruter]

In 41860:

Customize: Fix positioning of device-previewed window when publish settings are open.

• Update date control padding.
• Collapse publish settings section when opening panes for available widgets or nav menus.
• Remove save and cog buttons in mobile preview only mode.
• Move definition of wp.customize.previewedDevice to wp.customize.state('previewedDevice') for centralization, maintaining old alias.
• Remove unused CSS because the section is inside form now.

Props sayedwp, westonruter.
See #39896.
Fixes #42027, #42199.