[crud] Enable configuration if deletion is allowed

[Peppe]

In some cases, people have access to update existing items, but they are not allowed to delete them. For this, it would be great to have an API to disable the delete functionality.

[javier-godoy]

As a workaround, you can set a theme on the Crud component (let's say "read-only"), and style the shadow dom of vaadin-dialog-overlay as:

 :host([theme~="read-only"]) vaadin-button:last-child {display:none}

[TatuLund]

Enhanced CRUD adds getters for Buttons, so that you can set buttons disabled / hidden / change captions via Java API -> https://vaadin.com/directory/component/enhanced-crud/overview