visual should validate keys/values

Currently, the ``.visual`` property is just a free-form dictionary which allows things such as:

```
region.visual['banana'] = 'apple'
```

We should make sure we validate the keys/values for this dictionary and make it easy for users to find out the keys that can be set (perhaps by setting some sensible defaults)

Start by locating the `.visual` property and examining how its dictionary currently accepts arbitrary keys and values. Determine the supported visual keys and values from the existing package behavior or project discussion, then ensure invalid entries are rejected and users can discover valid settings through sensible defaults; done means the requested validation and discoverability are covered by tests.

Written by the indexing model from the issue text.