PropertyIO is declared in ReadOnlyProperty

[samreid]

As @pixelzoom described in phetsims/tandem#285 (comment)

(1) PropertyIO is not defined in Property.ts, and is in fact associated with core type ReadOnlyProperty. So it violates the PhET naming conventions, and was therefore confusing. If conventions had been followed, I should have been looking at replacements of Property.PropertyIO with PropertyIO.

At the moment PhET-iO does not distinguish between PropertyIO and ReadOnlyPropertyIO. Instead, everything is PropertyIO and non-settable ones are identified via phetioReadOnly: true. I do not know whether this is the clearest, best strategy, but just wanted to write down how it works at the moment. How can we decide whether we need to create ReadOnlyPropertyIO and reveal it to clients?