Minor improvement for Plant3DEditor extensibility

* AbstractVTKNodeMap needs an overridable postCommit() method which is
  executed after doCommit() finishes successfully. This is needed in an
  extending editor to know the first moment in time when the editor has
  been initialized properly.
* Allow customization of FitToWindow action created by Plant3DEditor

gitlab #117

Change-Id: I50a36740e579664ddab83b659b79220b5d33c135