Add a revision UI for subqueues

+1 This simple patch works and adds a useful feature to entity queue.

Old patch does not apply to new beta branch.

Here is a new patch with full revision support, including UI revision table and revert functions.

Permissions are not thought through, but works fine with administrator privileges.

I agree that revision support is very much needed, and now that Drupal 8.7.0 added an Entity Update API which can safely convert the schema of an entity type from non-revisionable to revisionable even when it has existing data, I think we're finally able to do this conversion safely.

Since multilingual support is also needed among other things, I opened a separate issue to do all the conversions in one go (#3058127: Convert subqueues to be revisionable, translatatable and publishable), and credited both of you for the work done in this issue.

That patch enabled revision support only at the API level, so I'm keeping this issue open for the UI part :)

A generic revision UI will be provided for us by Drupal core itself when #2350939: Implement a generic revision UI will be fixed, and I don't think we necessarily need it now in Entityqueue itself.

@ #6 there will still be glue required by this module. It wont be fully automatic. See change record https://www.drupal.org/node/3160443

Consider a new major/minor version requiring Drupal 10.1, now that the required revision foundation is merged: #2350939: Implement a generic revision UI

Considering that we don't even have a stable release of Drupal 10 at this time, an Entityqueue version that would depend on 10.1 is quite far off in the future :)

A we have a stable release of Drupal 10 un-postponing it.