Hierarchical Clustering: add legend for Color-by variable

**What's your use case?**
If I select a variable in the Color-by drop-down menu, I would like to have a straightforward way to find out which color corresponds to which category or range of the selected numerical variable. Especially if there are a lot of categories or if the selected variable is numerical.

**What's your proposed solution?**
Add a legend, just like in Scatter Plot and many other widgets.

**Are there any alternative solutions?**
Add Color somewhere upstream, so that I can customize the colors and know which color corresponds to what. However, in the case of a numerical variable I still won't get insight into the discretization.
Or: add a dummy Scatter Plot, select the same variable for coloring there and check out the legend - which is rather awkward.

Start at the Hierarchical Clustering widget and compare its Color-by handling with the legend in the Scatter Plot widget. Done means users can see which color maps to each category or numerical range, including the numerical variable's discretization.

Written by the indexing model from the issue text.