Correctly reset `prev` and `next` pointers of action group FCurves when
separating them into distinct `ListBase`s per `bActionGroup`.
These `NULL` pointers are necessary to temporarily demarcate the start &
end of the `bActionGroup::channels` list. Having them still point to
other FCurves caused ordering issues when moving curves towards the
start/end of a group.
This commit corrects the above issue and adds versioning code to rectify
any ordering issues that may have been caused. For this purpose the
`BKE_action_groups_reconstruct()` function is rewritten to avoid relying
on the `bAction::curves` list order or `prev` link integrity.
Differential Revision: https://developer.blender.org/D11811
