Next steps after the scatterplot PR

[LucaMarconato]

#270 is now merged! 🥳 Thanks again @fjorka and @melonora.

In the PR discussion were some open points to consider in follow-up PRs. I moved the points here, to make them more discoverable.

[LucaMarconato]

Small

• Flip the Y-axis (scatterplot in spatial coordinates is flipped relative to the image).
• Signal point highlight to the main viewer for quick connection between visualizations in different dimensions.
• Highlight object in the main Napari window to trigger highlight in the scatterplot.

Medium

• Performance improvements (1. How points belonging to polygons is checked.)
• Deleted, it will be done implicitly by addressing the task below new screenshots for the notebooks [to show the new border color] (after approving the new look by @LucaMarconato )
• for a follow up PR: replace the dataset used in the notebooks with the lightweight mouse liver dataset available here. in doing this, the new plots will show the correct border color (it has been updated).

Large

• Integrate Annotation Widget functionality into the Scatter Widget (the Annotation Widget facilitates annotating multiple classes within a single column).
• (possibly) add a warning dialog about overwriting sdata object