This came up in a recent #animation_rigging meeting.
Previously only keyframes themselves were supported in Graph Editor
snapping, now suport this for hadles as well.
As an example, only the `Selection to Current Frame` is supported here
to gather feedback.
Handles will only snap if their corresponding keyframe is deselected
(otherwise, we'd snap all into one place - which is not what we want I
assume).
ref T94174