The previous algorithm was buggy and did not handle the case when multiple VCs are being dismissed. Unfortunately I couldn't find a reliable way that'd allow for reshuffling modally presented VCs (inserting or deleting VCs not from the top) and so I added a special check that'd throw in the case someone attemted to do this.
On iOS by default modals show up in full-screen mode. This behavior can be customized and instead of mouting new screens in top level window they can be mounted under a given UINavController. We used to be relying on that behavior (see "CurrentContext" presentation mode). This, apparently wasn't matching the default functionality of the OS. This change is adding it as a default and keeping the old way under newly exposed presentation modes: containedModal and containedTransparentModal
Thanks to this change, iOS 13 modals can work properly.
