fix:Panel2D works with only size provided, no borders

[suchiii29]

Hey! I was going through the issues and found that Panel2D crashes
when you create it with just the size and no border arguments.

The problem was that self.borders was never initialized when
has_border=False, so _get_actors() and _update_actors_position()
were failing trying to access it.

I fixed it by:

• setting self.borders = {} when has_border is False
• adding has_border checks before accessing borders in the two methods

Tested it locally and it works fine now.

Fixes #1155

[VanshAgarwal24036]

Nice catch on the issue and fix.

One suggestion: it might be helpful to add a small regression test to verify that Panel2D works properly when has_border=False (e.g., adding actors, updating positions, and adding the panel to a scene).

Also, I noticed the PR removes the commented TabPanel2D class. Since this change is unrelated to the issue being fixed here, it might be better to restore that commented section in this PR.

[suchiii29]

Thanks for the review! I've restored the commented TabPanel2D section. I'll add a regression test for the fix too, working on it now.