Implement API to determine if owned object need to be published

[maxime-rainville]

As a developer, I want the capability to determined if they are any outstanding changes on a child object that need to be published.

Context

@mfendeksilverstripe has already implemented this in a PR #328. This issue is just backfilling.

Note that "owned" in this context is a bit ambiguous, as the PR includes cascade_duplicates in its concept of "owned".

Acceptance criteria

• Versioned as a new method that can be called to determined if child object need to be published.
• Owned "children of children" are considered when making a determination of the state of a parent object ... even if the first child is unversioned.
• Any "compromises" that are needed to make this API shippable in a minor release are documented and a follow up card is created to remove them in the next major.

Notes

• For context, ownership here refers to the private static $owns array.

Edge cases

• Calling the stagesDifferRecursive on a DataObject in the LIVE stage.
• Do we care about "unversioned" DataObjects that owned other DataObject?
• What happens for "versioned" DataObject that do not use stages?

Pull requests

• 328

Parent epic

• 419

[GuySartorelli]

PR merged. The API will be available from CMS 5.2