There's a new selection-box on the header of the action editor, which
sets the mode of auto-snapping for transforms. By default auto-snapping
is off.
There are 3 modes of auto-snap:
* Off - transforms per normal
* Frame Step - grid-step transform (may have errors with scaled actions)
* Nearest Frame - true snap-to-frame (takes into account nla-scaling)
These translate to the following hotkeys when transforming:
* Off - no keys press/held (as it's always been)
* Frame Step - Ctrl (as it's always been)
* Nearest Frame - Shift (replaces old shift-key behaviour which was not useful)
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