Allow grabbing a spline between two curve points to respect the curve point types.

This means that the BCP of the splinepoints on either side of the
spline we are moving are going to be touched so that they effect the
other corresponding BCP for the corner point and will force the
adjacent splines to update themselves properly in response to your
mouse movement.

Dave's video comment a few down on the below issue shows the desired
adjacent spline movement:
https://github.com/fontforge/fontforge/issues/533