ENH: Update VTKv5/VTKv6 to include vtkCornerAnnotation API improvement

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Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date: Tue Feb 10 19:43:30 2015 -0500

vtkCornerAnnotation: Add convenience TextPosition enum

Note: This changed has been submitted on gerrit for its integration
into VTK  master. See http://review.source.kitware.com/#/c/19191/

This commit extends the API of the corner annotation by introducing
an enum named "TextPosition".

Considering that enums are also wrapped in Python, this will improve
the readability of both python and c++ code.

The words Lower{Left|Right} and Upper{Left|Right} have been selected
to be consistent with vtkLabelPlacer API

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git-svn-id: http://svn.slicer.org/Slicer4/trunk@23970 3bd1e089-480b-0410-8dfb-8563597acbee