ROI: Remaining tasks and future improvements

• No need to limit to a single detector --- just make a Layout (Column)? Delay until we figure out how we really want to support creation of plots in a layout!
• When the frontend restarts it will not have the ROI-readback (since it is only published on change). Any active ROIs will thus be removed (after showing the curves briefly). This is not ideal, but unclear how critical in practice (hopefully we do not restart too often!).
• We have a fundamental problem with "stream ending". Currently there is no mechanism for this, one just has to stop publishing... and the frontend needs to figure out what to do. Concrete problem here: When deleting a ROI the corresponding (not unchanging) curve will still be in the DataService and may not be removed from the spectrum plot (it is from the plot where it was removed, but not in another observing plot). -> Fixed by tracking active ROIs via readback. Spectra are still in DataService, but removed from the ROI plot.
• Add options to normalize spectra from ROI to (1) pixels in ROI and (2) time. (1) will make comparing spectra from differently-sized ROIs easier.
• Support Polygon ROIs.

The issue names frontend ROI readback, DataService, spectrum plots, and Polygon ROIs but no files or tests. Start by tracing existing ROI readback and plot-update handling, then define separate completion criteria for restart recovery, stream cleanup, and spectrum normalization by ROI pixels and time.