feat(docx): add checkbox parsing support (#3349)

* feat(docx): add checkbox parsing support to MsWordDocumentBackend

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* refactor(docx): remove duplicate code in text element handling

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* docs(docx): update checkbox method docstrings

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* refactor(docx): use self._BLIP_NAMESPACES for w14 namespace in checkbox methods

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

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Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>