tvtk Window Position is of wrong type on initialisation (VTK 6.2, 6.3)

Tested VTK 5.8, VTK 6.3 and VTK 7.0 on Linux with Python 2.7
The following occurs only for VTK 6.3

```
>>> from tvtk.api import tvtk
>>> window = tvtk.GenericOpenGLRenderWindow()
---------------------------------------------------------------------------
TraitError Traceback (most recent call last)

...[skip some lines, see full traceback in the following link]...

in ()
----> 1 window = tvtk.GenericOpenGLRenderWindow()

TraitError: The 'position' trait of a GenericOpenGLRenderWindow instance must be a string, but a value of (0, 0) was specified.
```

See complete traceback here, as is captured by tvtk nose test:
https://travis-ci.org/enthought/mayavi/jobs/124533392#L863

After some investigation, this is what caused it:

```
>>> import vtk # VTK 6.3 from vtk.org
>>> import tvtk.vtk_parser
>>> parser = tvtk.vtk_parser.VTKMethodParser()
>>> instance = parser._get_instance(vtk.vtkWindow)
>>> instance = GetPosition()
'_0000000001f02788_p_void'
```

Whilst with VTK 5.8 and VTK 7.0 (Python2.7)

```
>>> import vtk # VTK 7.0 from vtk.org or VTK 5.8 from ubuntu
>>> import tvtk.vtk_parser
>>> parser = tvtk.vtk_parser.VTKMethodParser()
>>> instance = parser._get_instance(vtk.vtkWindow)
>>> instance = GetPosition()
(0, 0)
```