[Example] Modify list and form of built-in category entity

[niklasnatter]

What's in this PR?

This PR demonstrates how to modify the list view and the form view of the the built-in category entity. The example hides the key property and adds an additional color property to the list view and form view of the category entity.

To implement the color property, the PR extends the category entity and adds a Symfony event listener to persist and serialize the data of the new property.

To apply the changes, the schema of the database needs to be updated by executing bin/console doctrine:schema:update --force.

[niklasnatter]

Sulu automatically merges form-configurations that use the same key. This allows to hide existing properties and display new properties in existing forms.

To use this mechanism with form-configurations that include <section> tags, the workaround described in sulu/sulu#5570 might be helpful.

Be aware that newly added properties need to be persisted to the database and included in the responses of the respective API endpoints. For example, this is possible by implementing an event subscriber similar to the CategoryEventSubscriber in this PR.

[niklasnatter]

Sulu automatically merges list-configurations that use the same key. This allows to hide existing properties and display new properties in existing lists.

[rflorent]

Hello,
how to get the new property color on frontend when categories are used with pages ?
Thanks

[matthiasseghers]

@rflorent

You can get this by passing additional data to the controller.

[FlxRobole]

Thanks a lot! It would be quite interesting to know how to access the new color property in the items returned by twig-function sulu_categories (or when used via content-type category selection). As far as I can see the fields are mapped by CategoryWrapper which is called by CategoryManager::getApiObject(), but I have no idea how to "replace" or override the CategoryWrapper with my own (extended) wrapper. Only way out seems to be adding my own sulu_categories_with_colours twig-function...?!