bnc#885548: Initial work on importing revisions from xlsx.

Change-Id: Ie0528490d024093cbabf38541fe70be96a9caa2e
bnc#885548: Adjust xlsx export of revisions to get it to work in Excel.

Change-Id: I0058d9ddfea545390e615a3030171a366e333c85
bnc#885548: Add test for import / export of revisions for xlsx.

It's the same test as with the xls variant.

Change-Id: Ice3d3c34213cc521e2782ef0f98d28d135c72fbe