Minor section commenting.
Refactor implementation of `hasChildViewController:` to use `indexOfChildViewController:lenient:` as base.
Harden and expand pre-emptive adding of siblings for focus-pushed vc's.

Siblings are predefined through identifiers. A
next-sibling just gets pushed, while logic for a
prev-sibling is more complicated, involving
conditional removal of existing sibling and
conditional insertion, and so it is only added
now.
Fix simultaneous gesture recognition guarding.
Add optional focused-navigation scroll-view resetting.

Requires animated navigation.