writerfilter: DateFormat member in SdtHelper as a string.

Change the type of DateFormat member of SdtHelper class to a string,
and move some logic from DomainMapper to SdtHelper.

The string will be useful to improve the date control exporter later.

Change-Id: I0da05a59e15143027f1c5805466e9f465d7ac069
ooxml: preserve dateFormat for unchanged date controls

When saving a date control in a docx document, we were overwriting its
date format with "dd/MM/yyyy" because it's tricky to support all the
possible combinations. Nonetheless, there is no need to overwrite it
if the date in the control remains unchanged during edition.

We preserve the original date of the control, its date format and the
formatted date string on import, and we compare the date in the
control with the original one on export to check if we can write the
old values or we have to re-generate them. Only in case the date has
changed the format will be reset to "dd/MM/yyyy".

We had to add an InteropGrabBag field to the XControlShape because it
didn't have one, unlike other shapes.

Unit tests were modified to check that the dateFormat field is
preserved unchanged.

Change-Id: I01e5c990e90ff190b5a6d7ea3853e049ff24ef0a
ooxml: preserve locale attribute in date controls

It is stored in the shape grab bag on import and saved back on export.
Defaults to "en-US" if not present in the bag.

Added a check for this attribute in the unit test.

Change-Id: Ib9c74f120628dbfc5e035500424cbad8caed53fe
ooxml: export combo box controls

Export combo box controls as <sdt> elements. Added unit test.

Change-Id: I9eda25101240ec61f78ede0f0462d3ab8e77b09b