Blocks: Add a way to manage block categories

[gziolo]

Description

Supersedes #1732.

With this PR, It is possible to filter the list of default block categories using the block_categories filter. You can do it on the server by implementing a function which returns a list of categories. It is going to be used during blocks registration and to group blocks in the inserter. You can also use the second provided param $post to generate a different list depending on the post's content.

add_filter( 'block_categories', function( $categories, $post ) {
	if ( $post->post_type !== 'post' ) {
		return $categories;
	}
	return array_merge(
		$categories,
		array(
			array(
				'slug' => 'my-category',
				'title' => __( 'My category', 'my-plugin' ),
			),
		)
	);
}, 10, 2 );

How has this been tested?

Renamed the category of one of the blocks to my-category and included the code shared above to activate this category.

I also tried what happens when some existing categories get removed. You see warnings (expected non-destructive errors triggered by our code) on the console informing that blocks couldn't be registered because a given category doesn't exist. Which is more than expected.

Screenshots

Types of changes

New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[mcsf]

LGTM!

[aduth]

FYI this example is not PHP 5.2.x compliant (with use of closure, vs. named function).

[gziolo]

Yes, phpcs complained when I pasted it into codebase. I will split it into 5.2.x style before merging.

[gziolo]

Done in e75c4f3.

[aduth]

Should we want to keep the DEFAULT_CATEGORIES in the client?

[youknowriad]

That question will come up with the blocks as well. I think we need a way to generate scripts bundled as separate modules in case someone wants to use Gutenberg fully client-side.

[aduth]

I think it seems fine to have a default set in the client. From purely a maintainability perspective, it would be nice if we could avoid maintaining two separate sets of the same values though (e.g. somehow inject into the client JS from the server default value, or vice-versa).

[gziolo]

Yes, I thought about that, too. It would be nice to have it in one place, but it is tricky because of translations. I opted for duplication to move on. I open a follow-up issue where we can discuss how to tackle the standalone case.

[youknowriad]

There's already some previous work done by @aduth bin/get-server-blocks.php that can achieve something close to what we need.