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Change-Id: Ib936712a8dd49a7130057aebf0ba529e65eb2c66
unotools: improve date parser to support timezones

ISO8601 defines timezones with formats like 'Z', '+01', '-08:30' or
'+0830' at the end of the datetime string, but our parser was failing
when the string contained any timezone information.

I have modified the parser so it doesn't fail in case of the string
contains a timezone. Moreover, I check that the timezone definition is
correct according to the standard [1] and I extract it to the string
tokTz.

Unfortunately UNO Time class doesn't contain any field to store the
timezone, only the boolean field IsUTC, which I fill accordingly.

TODO: add a timezone field to UNO and fill it from the parser.

[1] https://en.wikipedia.org/wiki/ISO_8601#Time_zone_designators

Change-Id: I9d33b19efbc6d41a74b02ece2dfa12fa12fac5eb
unotools: extractDate and extractTime functions

Added a pair of functions to split a UNO DateTime object into the
corresponding Date and Time objects.

Change-Id: I6bf31d83b3843a9482d6bf538a5dd7130c2c7046
writerfilter: import date into docx date controls

Date controls specified by <sdt> tags in docx documents were being
imported but the date was being discarded.

To import the date, we added an attribute to the CT_SdtDate resource
in the ooxml model file to be able to read the date ISO definition
contained by the fullDate attribute. We store it in the SdtHelper
class and we transform it in a Date when the control is created.

Change-Id: Iac2d4f99c31188848c109fafb54121bfcdde060d
ooxml: export date controls properly

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
ooxml: fix export of empty date controls

The text contained in the <sdtContent> tag acts both as a help message
to the user and as the actual data; for example, Word 2010 is able to
set the date of the control from the text in the content if the
w:fullDate attribute in <date> tag is not set. For that reason we
should not use the sdtContent as the "HelpText" property of the
control in any case, only when it is empty.

The name of the parameter rDefaultText was modified to reflect that
string does not represent any default text, only the content of the
<sdtContent> tag.

Change-Id: Ic10334a949969ec8c7c3e25f73fb937a50ee0f7f
TODO: is it worth to internationalize the help text?