Factorize OnCenter/OnCorner/CornerIndex properties

[finetjul]

The msvVTKButtons properties OnCenter, OnCorner and CornerIndex are a bit confusing.

I would suggest instead to factorize those properties in 2:

enum LocationType
{
  OnDataCenter = 0,
  OnDataCorner = 1,
  OnScreenCorner =2
};
vtkSetMacro(Location, int);
vtkGetMacro(Location, int);

enum ScreenCornerType
{
  BottomLeft = 0,
  BottomRight = 1,
  TopLeft = 2,
  TopRight = 3
};
vtkSetMacro(ScreenCorner, int);
vtkGetMacro(ScreenCorner, int);

This would gives the ability to easily extend the list of possible locations.