In 30522626, next/previous key in SSequencer was consolidated to use FSequencerKeyCollection. However, SSequencer had explicit code that looked at section bounds (including those without channel groups). So that broke next/previous key for section bounds.
This change adds support for section bounds in FSequencerKeyCollection by traversing FSectionModels and not just FChannelGroupModels. There's now a map of SignatureToSectionBounds to cache the bounds.
Also, combined the functions to get next key and next section key. There's really no need to have separate functions to get next values and it just makes it more complicated.
#rb Mike.Zyracki
#rnx
#jira UE-207190
[CL 31593102 by max chen in ue5-main branch]
325d551b6c