Heat Map: enable display of categorial variables

**What's your use case?**

Social scientists often work with ordinal or binary variables, which are imported to Orange as discrete. It would be nice to enable the so-called _carpet plot_ in Heat Map - showing categorical variables with the color of the value as shown in other widgets.

**What's your proposed solution?**

Enable showing categorical variables in heat map.
Of course this would make it impossible to show a legend, so raise a warning saying "Legend not displayed for categorical variables". Perhaps we can think of a more elegant solution to this.

**Are there any alternative solutions?**
Have a new widget called Carpet Plot? But perhaps that's an overkill. Not sure.

Start at the Heat Map widget entry point and trace how imported discrete variables are currently handled. Compare the behavior with the requested carpet plot and warning; done means categorical variables can be displayed and the legend warning is shown when appropriate.

Written by the indexing model from the issue text.