Enable deletion

[SethPoulsen]

I assume people will want to be able to delete nodes and pointers somehow, even when in the mode where nodes that are left dangling are still drawn on the screen.

We need to come up with what the UI should be like, then implement it.

[jasonxia17]

If a deleted node has pointers pointing to it, what should we do with those pointers after the deletion? I think we should somehow indicate that those pointers are not null; rather, they are pointing to some garbage that we no longer have control over (because that represents what would actually happen in C++ in this scenario).

Figuring out how to undo a delete operation might also be tricky, since we need to restore the pointers that previously pointed to the deleted node.

[SethPoulsen]

Those are great points. I think part of the design here will end up depending on

1. the semantics of the language that the students are using along with the tool
2. if we want to restrict operations in order to teach something

Right now its looking like we are going to end up targeting students have mostly a Java background, so that might make the design a little different.

For now I think I'm going to wait on this until we have a more clear idea of the exact tasks we are going to be asking students to do.