Add expected type ItemList in some common cases

[danbri]

There are a few places where we ought to explicitly say "this property can have itemlist values".

• acceptedAnswer (some sites support several)
• suggestedAnswer
• step (on HowTo)? not needed since HowTo is a subclassof ItemList and ListItem ...
• http://webschemas.org/HowToSection should be a ListItem as well as an ItemList.
• http://schema.org/mainEntity ? not needed since it expects Thing

[kaylareedhanson]

Your change includes all the changes mentioned here, I think this can be closed?

[RichardWallis]

Released in V3.4