ODPresentation : a round trip keeps the kind of a field and loses which one it is

[dkulyk]

#959 let a field sit inside a sentence and
covered both formats. What it did not close is the variant: OOXML numbers fourteen dated formats
and five file formats, OpenDocument names the field by what it is, and the ODPresentation pair
writes nothing to carry the difference.

Measured on master, one field per deck, written and read back in each format:

type	ODP	PowerPoint2007
slidenum, slidecount, datetime, author	back as itself	back as itself
datetime3	datetime	back as itself
datetime11	datetime10	back as itself
file2	file	back as itself

Four of seven survive the ODP round trip; seven of seven survive the PowerPoint2007 one.

Both losses have a place in the format to live

• The fourteen dated formats belong in a number:date-style that text:date / text:time point at
  with style:data-style-name. Writer/ODPresentation/Content.php writes no data style at all, and
  Reader/ODPresentation::FIELD_OOXML says as much in a comment -- text:time maps to the flat
  datetime10 because which of the timed formats it is travels as a data style the Reader does not
  read.
• file, file1..file4 are the four values of text:display on text:file-name -- full path,
  path, name, name and extension.

Cost. A deck that goes PPTX -> ODP -> PPTX comes back with its dates reformatted. Nothing
crashes and no field is lost, so this is a fidelity gap rather than a defect, which is why it is
filed on its own rather than as a reopening of #959.

Work. The date half wants a number:date-style per distinct format in
office:automatic-styles, which is the shape the style pool of
#936 already handles -- it is on master,
so neither half is blocked. The text:display half stands on its own.

PR to follow.