Add control which provides buttons to edit various sidebars

[westonruter]

Easy access to editing the widget areas on a given template would be a key usability improvement. Currently a user has to navigate up to Widgets panel and into the widget area, and then all the way back to the page/post section after finishing. Instead the user should be able to click a button to open a given sidebar section, and then when that section is collapsed, the post section should be re-opened to return the user to where they were.

For the mechanism for using panel back button for breadcrumb navigation, see https://core.trac.wordpress.org/ticket/32683#comment:6

See also https://github.com/xwp/wp-customize-content-widgets

[westonruter]

The buttons should have their visibility state linked to the active state for the corresponding sidebars.