Inserter: fix the tabs focus style

[swissspidy]

Currently the focus style looks like this:

The tab with the focus key needs some z-index love here. Even z-index: 2 already works.

Note: I can't seem to be able to trigger it with the keyboard, because pressing tab automatically goes to the blocks list. See #1606 (comment) for a possible solution to that.

[josephfusco]

I can't seem to reproduce this.

Tabbing through seems to lock the focus into an infinite cycle within the current tab content with no obvious way on how to change tabs.

[afercia]

Tabbing to the tabs is currently broken, known issue and pending a final decision on the inserter functionalities to be fixed.

Accessibility of the inserter worked at some point (there were no tabs) see #527. Then the introduction of tabs didn't address keyboard navigation. The inserter has seen other changes under the hood, I hope it will be in a stable state soon so we can address accessibility.

[afercia]

See also #1497 (comment)

[jasmussen]

Pretty sure this is fixed by #3080.