Fixes qupath#1674
Also add "Non-rectangles only" as an option when sending ROIs to ImageJ, since rectangles tend to correspond to the entire image region... and do more harm than good.

Addresses qupath#1674
Don't convert a PolygonROI (QuPath) to a PolygonRoi (ImageJ) unless the polygon is simple (1 outer ring, no holes); instead, use a ShapeRoi for consistent results.

Several minor fixes, including
- use Roi group names for classifications, where available
- rename setting the plugins directory to 'Set local ImageJ directory' - because it's needed also for LUTs as well

Note that setting the Roi group logs an error if this hasn't been done, because it makes a request for the Glasbey LUT that can't be fulfilled.