Violin Plot with data subset

**What's your use case?**
I would like for the Box Plot widget to have the option of `Data Subset` like it is in the Scatter Plot widget. This option would add points to the drawn box plots to show where on the distribution the specific subset lies.

A simple mockup:

Start by locating the Box Plot and Scatter Plot widgets and comparing how the Scatter Plot handles Data Subset. Define the subset-point behavior from the issue and mockup, then verify that selected subset points are shown on the corresponding box plots without changing the distribution.

Written by the indexing model from the issue text.