MNT: Decide how to show readbacks in LivePVTableModel

[tangkong]

Current Behavior

If an Parameter with a linked readback Parameter is added to the table model, only the parent entry is added (since it was directly added to the model.

Expected Behavior

We should figure out a way to intelligently display parameters and their readbacks, showing relations between the two.

An extension of this: If a parameter is added with a readback that already exists in the table, what do we do? 🤔

Context / environment

Thought about during #80

Steps to Reproduce (for bugs)

N/A

Suggested Solution

Check the list when calling set_entries?
Figure out some slick way to have a foldable/nested view in a table model? 💥