Contextual links for custom blocks: Add "Edit block content" has somebody an idea, how to customize the contextual links in D8?

I have created a custom block type and it would be wonderful to have a link in the contextual links menu: "Edit block content".
The missing "edit" link was a bug in early Drupal 8 betas, and has been fixed as of beta5:
https://www.drupal.org/node/2284917