add "shapes" to plot

## Use Case

Users need ways to add dimensionality to the plot. The single use case we observed was: XY axes are set to umap values. Color by is set to "time" aka frame number, so that one can easily identify frames that are closer to the start or end of the time series. Then they want to also distinguish a "experimental condition" parameter which has only 3 or 4 possible values.

## Solution

For categorical features with relatively few possible values, offer the ability to display the points as shapes (e.g. triangle, square, circle, diamond etc). Needs some ux regarding how exactly to inform the user what is what (some kind of legend displayed maybe in the left panel)

## Alternatives

Alternative considered was 3d plot where we use height as the additional dimension but this introduces too much new code and new visualization modality into CFE. Deserves a different conversation.