Customize: Make the Add a Widget button a button

[afercia]

For semantics, accessibility and consistency with the Menu Customizer, the "Add a Widget" button should be a button element. See: https://core.trac.wordpress.org/ticket/33260#comment:30

we should switch it to a <button> element and move it into customize-controls.css.

Related: #33260 and #33327

[afercia]

Thanks for fixing the typo Sergey, you beat me to it for a few seconds :)

[metodiew]

I'm not sure if the CSS was moved properly. Also not sure if we need a context for the button i18n, so critics and suggestions are very welcomed :)

[afercia]

There are a few more things to do here. The buttons: "Add a Widget", "Add a Menu" and "Add Items" are very, very, similar and should basically have the same CSS rules.
Same for the HTML part, but here there's no need for an aria-label attribute to better specify the button's purpose. Also, no more need for tabindex.
About the JavaScript part, buttons don't need a keydown event. We should handle the aria-expanded attribute and set proper attributes when the UI is in "reordering" mode, as done in the Menu Customizer.

[afercia]

First pass with a more complete approach.

[afercia]

This ticket kinds of overlaps #33327 where the initial patch already took care of the "Add a widget" button too. It's basically a duplicate :) Just refreshed patch on #33327 and would propose to close this one.

[afercia]

Closing as duplicate of #33327. Will handle props in that ticket.