DOC export: allow saving as a template

Change-Id: I26894edd3962c2b2856d2bfa4a4aa4e6f4e65c9e
filter: mark writer_MS_Word_97_Vorlage as preferred

The other filter associated with the .dot extension is
writer_MS_Word_95_Vorlage, obviously the 97 one should be preferred.
With having one filter as preferred for this extension, plain
'--convert-to dot' starts to work.

Change-Id: I7207d0fcb3c55b8dcc7b6179ccfa33a7c85d2ce8