write OOXML externalReferences, externalLinks, fdo#45286

This for the first time writes external references (hopefully) correctly
and adds the necessary relationship streams and the externalLink streams
with sheetData. At least Excel 2013 loaded the result without
complaining, so do we.

(cherry picked from commit 8c23a767d926d8d08213f5e2f8e81775c653cbd7)

Conflicts:
	sc/source/filter/excel/xelink.cxx

do not write empty externalReferences element, fdo#45286 follow-up

OOXML validation spewed
- (xl/workbook.xml:2 col:700) cvc-complex-type.2.4.b: The content of
  element ´externalReferences´ is not complete. One of
  ´{"http://schemas.openxmlformats.org/spreadsheetml/2006/main":externalReference}´
  is expected.

(cherry picked from commit 8c51532289db36337c4984baec1bf5a7ca9e40af)

really do not write empty externalReferences element, fdo#45286 follow-up

Xti records exist also for internal sheet references in BIFF. It is not
enough to check for their presence, at least one EXC_SBTYPE_EXTERN must
be present as SUPBOOK.

(cherry picked from commit e04a4d2d2c18134556bb4e001d497ca7d66c6828)

Change-Id: I3d615490a60c5420ae13c0bfc6297642d86a07b9
Reviewed-on: https://gerrit.libreoffice.org/10872
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>