Form check box extraction with XWPFWordExtractor

For test issues, we compare automatic filled word forms at least by extracting the documents' text. Therefore it would be great if the XWPFWordExtractor could also extract the values of form check boxes as text to be able to compare their status on text base.

This pull request implements the described feature. Form check boxes in *.docx format will be extracted as |_| for an disabled check box and |X| for an enabled check box.