Fix: Properly Maintain Selection of Keyframes When Inserting / Removing Hold Frames.

Related Feature: Preserve multi-frame selection when active frame changes. Useful
for preserving the users selection on scrub, but also less error-prone when dealing
with modifying the user selection while concurrent "switch-time" operation is running.

Insert Keyframe Bug:
BUG:404519

Maintain Multiframe Selection Wishlist:
BUG:388779