visual should validate keys/values

[astrofrog]

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)

[astrofrog]

Ideally visual should maybe even be a Visual class with property-based access (we could keep dict-like access for a while then deprecate it)

[keflavich]

:+1: but I want maximal easter eggs in this.

[adonath]

Changing .visual to a dataclass might make sense as well...