UI: Improve parallel / collect UI support

**Is your feature request related to a problem? Please describe.**
Parallel & Collect aren't appropriately marked in the UI and also don't have the right display and task linkage in the execution view.

**Describe the solution you'd like**
1. In the visualization view we should have similar markers like the graphviz visualization does.
2. In execute, we should be able to navigate upstream and downstream of tasks.

**Describe alternatives you've considered**
N/A

**Additional context**
I believe this just needs UI support, versus also requiring SDK changes.

Start by locating the visualization view and the execute view mentioned in the issue, then trace how Parallel and Collect tasks are currently rendered and linked. Compare the visualization behavior with the graphviz visualization and verify that execution-view navigation works upstream and downstream; done means both views show the correct markers and task links.

Written by the indexing model from the issue text.