Remove title attributes: the Featured Image postbox

[afercia]

See related #24766 and all the following tickets about title attributes.

When a post has a featured image set, the image link has a title attribute that says "Set featured image".
If this is a relevant information, it should be shown in plain text in the interface otherwise should be just removed.

Additionally, the focus style can be improved. See screenshot:

[afercia]

First pass:

• remove the title attribute
• fix the focus style
• add a new post label: items_list_navigation
• show the label in plain text in a howto paragraph, as done for example in Tags postbox

See screenshot with the patch applied:

About the new paragraph, open to suggestions for a better wording. Any thoughts more than welcome.

[SergeyBiryukov]

I'm not sure we need a new label here.

The point of #19257 was to allow CPTs to rename "Featured image" to something else. But "Click the image to edit or update" is pretty generic and should not depend on the post type (it's still an image, after all).

We could add the label later in a separate ticket if someone makes a request for that.

[afercia]

Thanks @SergeyBiryukov, cool. So: just a string, the simpler the better.

[afercia]

Refreshed patch: removes the useless new post label in favor of a plain translatable string.

[afercia]

In 36000:

Accessibility: Remove the title attribute from the Featured Image postbox thumbnail.

The title attribute is now replaced with plain text in a howto paragraph, as done for example in the Tags postbox.
Adds an aria-describedby attribute on the thumbnail link to target the description.
Also, fixes the focus style on the featured image thumbnail.

Fixes #35076.

[afercia]

When there's no featured image, the aria-describedby attribute points to a non-existing element, which is invalid code. It should be set following the conditional logic in _wp_post_thumbnail_html().

[afercia]

Prints out the aria-describedby attribute conditionally

[afercia]

In 36584:

Accessibility: after [36000] conditionally print out the aria-describedby attribute on the Featured Image postbox.

Fixes #35076.