When saving a file that is not in ODF format (the default format), there
is a dialog asking if should keep the original format or rather use ODF.

But the default format can be changed to a different one than ODF. And
the dialog will still assume the ODF is the default one.

This commit fixes the wording in the dialog to use the the default
format instead of hard-coded ODF.

In order to keep the text simple, instead of using the long name for the
default format, it uses the proper file extension.

So if for example changes from ODF to Microsoft Word 97 default format,
and saving a non-Word document, the dialog will ask user if document
should be saved in default DOC format.

Change-Id: I0b259af9eca61161ec8a3d7e72d4722b5afdf4b4
Reviewed-on: https://gerrit.libreoffice.org/15170Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>