Improve DAG Visualization

[mdr223]

1. The DAG should actually be a DAG (instead of a pipeline)
2. The boxes within the DAG should consistently be well formatted (like semantic filter, not like semantic groupby)
3. The boxes within the DAG should be editable to update the logical plan (e.g. updating predicates & column names, deleting / adding operators, moving / dragging operators).
4. The DAG should pop-up like the Gemini coding windows; once it's hidden, there should be a button to re-open it.