Change GridfieldDetailForm to be opt-in

[tractorcow]

Fixes #21

[tractorcow]

See silverstripe/silverstripe-framework#7295 for docs.

[unclecheese]

Code looks good, but the approach gives me pause. Something is really bothering me about the versioned_gridfield_extensions DataObject setting.

We all know DataObject is pretty monolithic and hosts a wide array of concerns, but it seems to me this is the first member of its API to speak directly to another class. Things like summary_fields and searchable_fields primarily used for GridField, but there's a very reasonable case for using them elsewhere. So I guess without getting into a huge philosophical discussion on one static, I'm wondering two things:

1. Is there better semantics we can assign this, so that it could conceivably be consumed by other APIs?

2. Is this the best place to store this setting? Just thinking out loud here, but what if we made it GridFieldItemRequest's concern, and registered versioned DataObjects with it, rather than the other way around?

[tractorcow]

We all know DataObject is pretty monolithic and hosts a wide array of concerns, but it seems to me this is the first member of its API to speak directly to another class. Things like summary_fields and searchable_fields primarily used for GridField, but there's a very reasonable case for using them elsewhere. So I guess without getting into a huge philosophical discussion on one static, I'm wondering two things:

It's not an API of DataObject; I didn't add any private static to dataobject.

Is there better semantics we can assign this, so that it could conceivably be consumed by other APIs?

It doesn't need to be consumed by other APIs. This is a setting for a specific extension.

Is this the best place to store this setting? Just thinking out loud here, but what if we made it GridFieldItemRequest's concern, and registered versioned DataObjects with it, rather than the other way around?

It would be a whole lot more complicated to do it that way so I chose the simplest way.