Fix DropdownMenu arrows navigation and add missing aria-label.

[afercia]

Fixes the DropdownMenu navigation with arrows.
DOWN and RIGHT arrows were working, UP and LEFT weren't. Not sure why the difference, but this is probably related to the recent introduction of arrows navigation through blocks.
If so, there are probably more places where stopPropagation() will be needed.

Also, adds a missing label on the button that opens the drop down menu: it was completely empty... Uses wp.i18n.__() for consistency with what is already used in the file.

Todo: when one of the menu items is focused, pressing Escape should close the menu but it also makes the whole toolbar disappear. Will keep investigating and open a separate issue for this, probably together with other a11y improvements.

Fixes #1874

[aduth]

Yeah, appears this is needed because of the behavior of onKeyDown and block navigation in VisualEditorBlock.

gutenberg/editor/modes/visual-editor/block.js

Line 341 in af63d4e

onKeyDown={ this.onKeyDown }

We could move onKeyDown handler to the block's immediate wrapper element (i.e. not handling events in toolbar), but since the root element in this component can receive focus, we probably want arrow navigation to apply to the entire thing.

Which is all to say, this is probably a good solution, though problematic that descendants of the VisualEditorBlock component need to be aware of the behavior.

[aduth]

Per #1332 (#1205), we should be explicit about dependencies on the i18n module, i.e. use import { __ } from 'i18n'; instead of assuming global.

[afercia]

Yep I know, I didn't want to change also all the other ones, because outside of the scope of this PR. But since we're here, I can do that if you agree.

[aduth]

I didn't want to change also all the other ones, because outside of the scope of this PR.

Sure, especially in smaller pull requests like this one, I'm fine with some general refactoring sprinkled in as well. In an ideal world we'd have separate individual pull requests to refactor, but alas the world is not ideal and I find such PRs are few and far between.

But agree with the sentiment generally 👍