Customize: Link `elements` prior to embedding to prevent possible errors when...

Customize: Link `elements` prior to embedding to prevent possible errors when a control is associated with a non-existent section.

Fixes issue specifically with attempting to access an orphaned control's `elements` immediately after it has been added. Normally this would not happen because a control would not be registered without a section, and also a control should only be interacted with once its `embedded` deferred has been resolved.

Also harden logic for gathering list of deferred setting IDs.

See #37964.
Fixes #42330.

Built from https://develop.svn.wordpress.org/trunk@42024


git-svn-id: https://core.svn.wordpress.org/trunk@41858 1a063a9b-81f0-0310-95a4-ce76da25c4cd