Masks api improvements

- Added MaskLayer.splines.new (which creates single spline
  and returns pointer to it).
- Added MaskLayer.splines.remove() to remove given spline.
- Added MaskSpline.points.new() which creates new point
  in the origin and returns pointer to it.
- Added MaskSpline.points.remove() to remove given point.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56973 954f8c5b-7b00-dc11-b283-0030488c597c