Properties with required properties inside

[danzhik]

Description

Add property fields for properties, which are schema types itself, because otherwise Google does not correctly read these properties (errors during structured data check)

Possible soluitons

1. Provide a user ability to fill in only required properties, which will prevent errors in SE, of the properties being types itself.
2. Provide a user ability to fill in all properties of the properties being types itself.
3. Simply not to use properties wich require properties for themselves

Steps to Reproduce

1. Create classes for ALL schema types from Schema.org
2. Change metaboxes creation
3. Change output creation