sw: Properly embed .xls sheets in .docx documents

Word requires exactly the right relationship and content types to open
embedded documents properly. The case of xls sheets is different from
xlsx so we added it to the exporter.

We know if the file is a xls or xlsx using the ProgID field:
  ProgID="Excel.Sheet.8"  -> Excel version 8  -> xls
  ProgID="Excel.Sheet.12" -> Excel version 12 -> xlsx

Change-Id: I80bae0eadd4af2fb9793c8f6581b3f288fb42c09