FIX Work with ManyManyThroughList

[dhensby]

This PR depends on silverstripe/silverstripe-framework#8065

This PR adds support for sorting via a ManyManyThroughList

fixes #254

[dhensby]

There are other instances in the class where we check if $list instanceof ManyManyList but none of those seemed to need changing to make sorting in the CMS work.

[mfendeksilverstripe]

@dhensby I tested your new changes on a setup which uses versioned Many Many Through Relation and it doesn't work properly. I noticed two problems here:

• GridFieldOrderableRows::executeReorder() has some custom logic for ManyManyList and is likely in need of custom logic for ManyManyThroughList as well (note that ManyManyThroughList does not inherit from ManyManyList), the issue I found is that the $current contains empty values
• GridFieldOrderableRows::reorderItems() handles versioned ManyManyThroughList as non-versioned which is definitely not good, have a look at how the $isVersioned is determined, you may need to add a new case for ManyManyThroughList

[NightJar]

@dhensby @mfendeksilverstripe - I have an in progress work of this as a solution to #25 - however there needs to be a bit more done in order to have it work well, thus there's been no PR just yet. If one or both of you could check it out I'd be grateful.

(see last comment on that issue for links to fork).

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WIP PR opened at #260

[tractorcow]

Merged silverstripe/silverstripe-framework#8065, so this PR can be re-reviewed.

[NightJar]

Still too shallow an approach, as @mfendeksilverstripe mentioned.
Notably the first line below each of the changed if conditions; $list->getExtraFields() which is not a thing on a ManyManyThroughList.

As such I saw fit to continue the work on #260 - which is now functional. @mfendeksilverstripe would you care to confirm please?

[dhensby]

replaced by #260