sax, xmloff: fix ODF import/export of text:time/text:time-value

The value written for an Impress time field is something like
text:time-value="0000-00-00T23:28:07" (in LO 3.5+) or
text:time-value="0-00-00T23:28:07" (in OOo 3.3) which contains an
invalid all-zero date.  Such values are actually rejected by the
ODF import since commit ae3e2f170045a1525f67e9f3e9b7e03d94f2b56b.

Actually there was no real support to read the RelaxNG type
timeOrDateTime before.

So fix that by:
- adding convertTimeOrDateTime/parseTimeOrDateTime functions to
  sax::Converter
- recognizing and ignoring the 2 invalid all-zero values written by
  LO 3.5 and historic OOo respectively
- writing a bare "time" in text:time-value if the DateTime struct
  contains zero Date members
  (Older OOo versions and AOO cannot actually read that, but everything
  they _can_ read is invalid ODF...)

(cherry picked from commit cc407e50e8a1a74f9d1ed29d444dce9bd2e9167a)

The backport contains one change:
XMLTextFieldExport::ProcessTimeOrDateTime() still writes the invalid
value (to not add new backwards compat issues in stable branch),
so this patch only fixes the import side of things.

Conflicts:
	sax/source/tools/converter.cxx
	xmloff/source/text/txtfldi.cxx

Change-Id: I754076caee74a5163ed3f972af0f23796aa14f9f
Reviewed-on: https://gerrit.libreoffice.org/7026
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>