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

Change-Id: Icdfab06556a332235ecc50f0b4d1d2f9888365be