Table model routine did not respect given absolut path

To handle failed tries to define a table model the file path gets stored
internally. The implementation allways used the curret working directory
to retrieve an absolute path and therefore it was not possible to write
the table model to a different location.

The function 'path_realpath' has been implemented to provide an effective
conversion between a relative and absolut path while respecting the
OS specific path identification. Using this function in the table
model routine solves the issue.