Replace repeated refresh with polling in empty state

[CtrlSpice]

App now polls trace store for number of traces every half-second and reloads when there's something there.

[jmorrell]

This is a much better experience! Do you think we could auto-show the most recent trace? or is that a further change?

https://user-images.githubusercontent.com/175496/216736399-d1a7237e-0390-4811-8cb8-da95ae1906ef.mp4

[CtrlSpice]

Auto-showing the most recent trace is the thing I'm doing in my next PR 😄