Blender Git Loki

Fix T38774: Changing extrapolation type via RNA doesn't update FCurve

- Added update callback to perform on-update validation when changing the
extrapolation mode on F-Curves
- There was a patch in the tracker for adding an "update()" method to F-Curves
which does a similar thing when manually called by scripts. Since we've added
a function for this in RNA anyways, we might as well add this too while we're
at it. (NOTE: upon closer inspection, the original patch by Tom Edwards had a
number of issues, so I ended up reimplementing here)