Make plot generation asynchronous to keep the UI responsive

[paulromano]

This change makes plot generation asynchronous so the UI stays responsive during potentially expensive calls into openmc.lib. It introduces a background worker thread for id_map/property_map, queues plot updates, and updates the view when results are ready. A status‑bar busy indicator translucent overlay on the plot provides feedback while plots are generating. Let me know what you think!

[pshriwise]

This is really nice. Being able to interact with the app window while plots are being generated is a huge plus.

I like the status indicator and status bar text idea, though the status bar indicator was a little subtle to me, particularly on a wider monitor. I tried combining the two into an overlay in the plot image area. What do you think?

Screencast.from.2026-02-05.15-23-34.webm

[paulromano]

@pshriwise Interesting, can you throw that on another branch for me to play around with? If I like it, I'll merge it into the branch here

[pshriwise]

Here ya go! https://github.com/pshriwise/plotter/tree/update-overlay

[paulromano]

I do indeed like your version better and merged it in; thanks! I made a few more small updates -- let me know what you think now.