Pipeline network diagram should be clickable

[jbednar]

At some point, the nodes in the pipeline layout diagram were clickable, selecting that stage.

Hover currently (in Panel and HoloViews master) works for the nodes, but clicking doesn't do anything. I think that clicking should be supported in at least the following cases:

• clicking on the previous and next nodes should work precisely the same as clicking on the Previous and Next buttons. Clicking on the diagram is a very clear way for a user to unambiguously select a stage, and supporting that also lets a panel designer suppress those buttons to save screen real estate without losing any functionality.
• Clicking on arbitrary stages should also be supported, if one could already just click forward or back to get there, and it should have the same semantics as doing so (being supported in all the same cases and having the same effect).
• Clicking on the start node should always be supported, even for a Pipeline that defines a ready_parameter, with results equivalent to reloading the page if moving backwards to that node is not otherwise allowed using the Previous button. I.e., it always makes sense to start completely over (right?), given that a user can already reload the page, so I think the diagram should allow that behavior. Otherwise it's frustrating for users -- they can see what they want to do, but no way to make the pipeline do it, even when what they want to do is inherently appropriate and reasonable.