`DashCanvasView` enhancements

[ghsolomon]

Discussed with @haynesjm42 - looking to make a couple enhancements to DashCanvasView and DashCanvasViewSpec for a client app:

• [ ] Add ability to specify a custom title renderer to display in the panel headers
• [ ] Add ability to specify classNames per view: ideally, these would be observable, so they could be set dynamically at runtime

[amcclain]

@haynesjm42 is going to revisit and add details on actionable improvements