[4.x] Supply field's config and form's handle to extraRenderableFieldData method

[martyf]

The key tweak here is supplying the form's handle to the new extraRenderableFieldData method.

This allows the potential for form-specific calculations to take place within a custom fieldtype.

Further to this, to prevent fields being rendered multiple times, the Form tag has been tweaked to collect the fields from the sections, if the form rendered any, otherwise fall back to the older way of doing it. Without this tweak, the fields get rendered twice per form.

[martyf]

Revised to pass the field instance to the extraRenderableFieldData method.

This means that the specific field's configuration can be used at this level - which really opens up capabilities.

[martyf]

Revised to actually only be the field's config - that's what is important here.

It allows the logic in extraRenderableFieldData to do things differently based on how the fieldtype instance is configured.

I know this changes the definition of extraRenderableFieldData but hopefully is not in major use at the moment, and can slide in quietly?