Checkbox controls in docx are implemented with a sdt block containing
the property w:14:checkbox, which has several child elements.

To implement the preservation of the field, we had to add several
tags to the ooxml model file. Notice they belong to namespace w14,
that's why we had to add a redundant definitions of CT_STRING,
ST_STRING and their actions. Then, we just used the existing
mechanisms on the importer and the exporter.

Finally, added a unit test.

TODO: the checkbox is preserved on export but it cannot be modified.
We should import it like we do for combo and date boxes.

Change-Id: I40f29b48ec24719c48482b9c8cff10176d1b188d