BUG #14283. Fix change events fired by vtkControlPointsItem.

This commit consolidates the logic between all vtkControlPointsItem subclasses
to consistently notify rest of the world that the control points were modified.
Previously, commit 1509669d87 tried to do the same with
vtkControlPointsItem::PointsModifiedEvent, however vtkCompositeControlPointsItem
wasn't firing those consistently resulting in the BUG #14283.

While attempting to fix that, I noticed that vtkControlPointsItem has
StartChanges()/EndChanges() methods that could be used for a similar purpose:
notify the world that control points are being changed. Hence, remved the
PointsModifiedEvent, instead fixing the code to consistently use
StartChanges()/EndChanges() calls.

Fixed StartChanges/EndChanges to fire the event on the vtkControlPointsItem
(along with tranfer function, as it was previously) and added logic to avoid
duplicable firing of events due nested calls to StartChanges/EndChanges.

Change-Id: I144e775cbdfbc9c1334cfc2b02609b37dda8cae7
Merge topic 'fix_control_points_editing' into master

a6b0b0df BUG #14283. Fix change events fired by vtkControlPointsItem.