Export date controls as ooxml Standard Document Tags (SDT) instead
of replacing them with plain text.

SDT date controls contain the date in ISO format as an attribute of
<date> tag, a custom date format that can be specified by the user
in the <dateFormat> tag and the date formatted in that custom format
in the <w:sdtContent> tag.

The unit test testFormControl from ooxmlexport suite was removed,
it only checked if the date control was exported as text and it
obviously fails now. A new test that checks the values of the
exported control was written instead.

A pair of date format functions were added to datetimeutils.hxx.

TODO: to avoid supporting all the posible custom formats that can be
specified in the <dateFormat> tag, it is forced to dd/mm/yyyy on
export.

Change-Id: I9d1b6f840ee9e133831fdb04ad399fe31bcb2063