Date SDT from DOCX is imported as date control since commit
3ec2d26d (bnc#779630 initial DOCX import
of w:sdt's w:date, 2013-04-30).

One detail I missed there is our date control is strict: it doesn't
allow free-form text. However, DOCX date SDT has an optional ISO date,
but the actual value can be free-form text. This means that importing
free-form text without an ISO date is lost on import.

Fix the data loss by restricting the creation of the date control: only
do this if we recognize the date format or in case we have an ISO date.
Otherwise just show the free-form text to avoid data loss.

Change-Id: I8125bdc749954a6a1c496de74b6682744adb7680
Reviewed-on: https://gerrit.libreoffice.org/63311
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>