* Started porting back keyframe editing tools for the Action Editor/Dopesheet. Currently, only Snap (Shift-S) and Mirror (Shift-M) are functional.
* Added keyframe-editing API method for ensuring that all IPO-curves are left in a valid state after modifiying the values of their keyframes.
* Added operator-register flags for most of the operators. Only mouse-select doesn't have it for now (as there's not much useful info stored, and no exec callback).
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