Subset marking on heatmap

[Hrovatin]

Some widgets (e. g. scatter plot) enable subset marking via Data Subset input. Could this be added to the heatmap? Namely, a subset would be marked with unfilled rectangle, as is currently done for the selection.

[janezd]

This is not the same as projections. I wonder whether it wouldn't be more useful (not to mention simpler to implement) if every rectangle would represent a different subset?

[janezd]

Will become unnecessary after #4443.