Coloring in OWdotMatrix

[JakaKokosar]

Also, and this may not belong in this PR, but would be an improvement, maybe coloring the dots according to their mean expression would make sense. Something like what scanpy does. If nothing else, it would look prettier.

Scanpy-s dot plot is slightly different: the size of the dot indicates in what fraction of the cells the gene is experessed in and the color indicates the mean gene expression.

Originally posted by @pavlin-policar in https://github.com/biolab/orange3-single-cell/pull/354#issuecomment-526951127