Nomogram: Lock points on Selected Data

**What's wrong?**

When Selected Data is on the input of Nomogram, one can still move points around. They should be disabled/locked, as moving them around causes confusion and makes no sense.

Also, what happens is more than one data instance is on the input? It looks like the last instance is considered, but one doesn't get any warning or anything.

**How can we reproduce the problem?**

File (iris) - Naive Bayes - Nomogram.
- Data Table (select a row)

**What's your environment?**

- Operating system: OSX
- Orange version: 3.32.dev
- How you installed Orange: pip

Reproduce the issue with iris, Naive Bayes, Nomogram, and a selected row in Data Table. Start at the Nomogram widget entry point and verify how Selected Data and multiple input instances are handled; done means selected points cannot be moved and the multiple-instance behavior is explicitly handled or reported to the user.

Written by the indexing model from the issue text.