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Add an option to do keyframe insertion in a cycle-aware fashion.

When enabled, inserting keyframes into F-Curves with simple cyclic
extrapolation (the same conditions as required for cycle-aware auto
handle smoothing to activate) will take the cycle into account:

- Keyframes that are being inserted outside of the cycle bounds
are remapped to be inside the cycle. Thus it is not necessary
to be within the main iteration of the cycle when tweaking.

This becomes especially useful in the final animation tweaking
phase when the channel keys may be staggered for overlap, so
the actual master period is different for different channels.

- Modifying one of the end points of a cycle also changes the
other end point when appropriate, to preserve smooth transition.

This feature applies to both manual keyframe insertion using
'I', and auto-keyframing.

Differential Revision: https://developer.blender.org/D3140