When registering actions, we've historically deferred them if they

depended on a parent that wasn't yet registered. With the new

multi-placement actions being introduced, it's possible an action would

be placed somewhere currently valid and somewhere that may never be

valid (such as a legacy attachment point or one defined by an

extension). When this happened, the action would never end up being

registered.
Actions are no longer ever deferred. Instead, placements depending on a

missing parent are now deferred.
When placing an action during registration or when resolving a deferred

placement later, any deferred placements depending on the newly-placed

action will be further resolved. This ensures that placements will

always resolve once all parents have been placed, no matter when that

occurs.
If an action is unregistered, any actions depending on it will move back

to a deferred state so they can be re-placed if the action comes back.

This is important for extensions.
Along with this, the registry has been updated to make use of

on_item_* hooks, instead of overriding register and unregister,

allowing the registry to manage when this logic occurs, taking advantage

of any locks.

All unit tests passed.
Verified this fixed issues with in-progress changes with actions

containing a placement that had a valid parent and a placement

that did not.