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 ae3e2f17.

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...)

Change-Id: I754076caee74a5163ed3f972af0f23796aa14f9f