Drupal 9 compatibility for Homebox

Would be nice to see this on the dev release.

Note that this would only make the module installable. At the current stage it's not working at all. I created a follow up issue here #3204838: Fix entity definitions & deprecations for D9 to fix most of the issues I could spot while trying out the module.

I updated the dependencies so that the separate jQuery UI modules can be loaded too.

It seems like the problem with the draggable system not working is that the SortableJS library doesn't work with the data structure that Homebox provides.

In every example of SortableJS I can find, it expects the "draggable" items to be direct child of the main element. Homebox nests the draggable items several layers deep, so it can't find the items. I've gotten it to mostly work by disabling the "draggable" setting, so that it automatically just uses the DIVs inside the columns.

The full solution would be to fix the HTML output.

What works:

• Reordering items inside a column.
• Updating the display and having it reload with the changes in place.

What doesn't work:

• Moving items from one column to another.

Just to clarify: The last part left is to work out how to make items draggable between columns.

@DamienMcKenna: Great work so far! Wouldn't you perhaps want to apply as Co-Maintainer of this project? Or is there a Maintainer for the Drupal 8/9 branch available to review and commit your changes?
See #3236974: Offering to maintain Homebox

The current Drupal 8 branch is old and broken... things can only become better...

@Maintainers: Could the current state of the @DamienMcKenna's work in the issue fork please be committed or at least be created as 2.x branch? Due to missing Drupal 9 compatibility in the .info.yml file, the current version can't even be tested in Drupal 9. Thanks!

Just to clarify: The last part left is to work out how to make items draggable between columns.

Just had a look at Layout Builder. It uses
https://github.com/SortableJS/Sortable

Perhaps that's the better choice here?

@Anybody: I only used the system for one project and I don't work on it very much, so I'd prefer to not take on that burden.

Created a new semver release: 2.0.x for future Druapl 9 development. Let's continue in separate issues for open tasks. If required I'll create a meta issue.

@DamienMcKenna: FYI: I was able to fix the drag & drop issue: #3247103: Items not draggable between columns :) Thank you for your important hints!
I was also able to remove the jQuery drag / drop libraries.