UI: Improve parallel / collect UI support

[skrawcz]

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.