Fix instance id shift bug; fix obsm viz; fix background color labels

[LucaMarconato]

Closes #242

Fixed:

• bugs with layer.properties:
  • on mouse hover the layer properties were not shown (this was visible in previous napari-spatialdata versions)
  • the correct instance property is now shown; properties were shown for the wrong instance, as reported in color legend for categorical annoation in table #242.
  • This is available for all the types of annotations obs, var, obsm and dataframe columns.
  • Available for points and labels. Not for shapes, see issue in napari: Properties shown in the bottom bar for labels and points, but not for shapes napari/napari#7306
• plotting a continuous variable on a labels caused:
  • the background color to be black instead of transparent; now it's transparent
  • the first instance (no matter which index, for instance 2) to be black, instead of being of a different color
• plotting variables on obsm was not working
• the coloring of categorical variables via pre-saved colors was not working because there was a "_color" instead of "_colors"
• there were some imprecisions in the tests for the scatterplot widget:
  • the table was not parsed using TableModel
  • an unused (and wrong) region_key parameter was passed; I removed it
• some modifications in the docs; in particular I warn the user that when zooming in on points in macOS one needs to be careful when looking at the values of points on mouse hover. In fact points that seem to be far away could still be overlapping. Example here: circles getting smaller when zooming in #35 (comment)

Missing (I will track these are new issues)

• tests for plotting data from obsm
• tests for hovering with the mouse and checking that the status bar of the viewer shows the correct value