Blender Git Loki

Fix T87173: wrong Auto-Snap in animation editors

This was partially broken with {rBde9ea94fc6f}.

The `Frame Step` and `Second Step` snapping options were working as if
they were `Nearest Frame` and `Nearest Second` respectively in the
`Dope Sheet` and `NLA` editors.

In the `Graph Editor` the problem was more serious:
"Second Step: ... The keyframe itself moves along as though in snapping
were active at all, while its handles 'stay behind' until it reaches
the next second boundary, at which point the teleport handles to
'catch up'".

The snapping code for these modes was spread across the transform
mode code and `recalcData` of each data type. Therefore, create a
unified snapping code for these options so that all issues are fixed in
one place.

Differetial Revision: https://developer.blender.org/D12241