[Form] Add the prop isReadOnly to the Form component

[nicmell]

The pull request #807 introduced the prop isReadOnly on the FormItem component to specify if the field is modifiable o read-only.
It would be desirable to add the prop isReadOnly to the root Form component to control the behaviour of every item in a single place.
The requirement suggests the implementation of a custom React.Context to hold the value of the prop, and having each FormItem reading from the context to infer its default read-only status (passing the prop directly to the FormItem should override the behaviour specified by the parent Form component).