T54233: NLA "Push Down" discards Blend Mode, Extrapolation, and Influence

Each AnimData block has a set of Blend/Extrapolation/Influence settings
that can be used to control how the active action is blended with the
NLA stack. However, these settings were not getting copied over to the
newly created strips (as the push-down code existed long before these
settings were added).

This commit solves this in several ways:
* Active Action Blend/Extrapolation/Influence settings now get copied
  to the new strips when adding them to the NLA stack via Push Down.

  Note: This doesn't happen when there are no existing NLA tracks,
        as these settings don't get used in that case.

* Strip Influence will be copied across when inf < 1.0 (i.e. when a
  non-default value is used), to maintain the effect. To make this work,
  the influence value will get added as a keyframe to the strip's
  "Influence" Control FCurve.

  - See code comments for an alternative approach and why that was not chosen
  - Strip Time still doesn't get keyframes added automatically yet.

* To ensure the "extrapolation mode" settings don't get always overwritten,
  I've put in place a compromise: the extrapolation will only get changed
  if the chosen setting will cause problmes (i.e. hold forward & back -> hold forward
  if there are other tracks before it already).

Not safe for backporting to 2.79[x] stable releases.

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