feat: Change the Hierarchy to support egui::CollapsingHeader

[MickHarrigan]

This is a quick change that converts the selectable_labels being used for the hierarchy to CollapsingHeaders.

There is still an issue with some entities that are brought in from .glb files it seems and I would love some feedback about that.

Otherwise I believe that this is just a small feature update.

[MickHarrigan]

Added a check for each entity to see if the children that it has implement the PrefabMarker that is required to be shown correctly in the editor.
This should (hopefully) fix the issue with the .glb files referenced earlier.

[rewin123]

Thanks for the pull request!
It's a nice addition. I'll check it today

[rewin123]

Looks almost like a working solution already. I only found a bug that when adding a child to an entity, the parent entity is moved one space along with the child. Only the child should be moved one space.

After that, the pull request will be ready for merging.

[rewin123]

I hope you don't mind, I fixed the described problem and got rid of code duplication. If you like my solution, pull request is ready to be merged with the main branch and will be in 0.3 version.

[MickHarrigan]

No worries on the edits and all, that was what I was looking for! Otherwise it looks all good to me.