Split pdf format as discussed on the list

Split pdf format as discussed on the list

Previously, the format used for included pdf files was the same as for
document export via ps2pdf. This caused unwanted conversion routes, e.g.
export via odt->pdf instead of dvi->ps->pdf.
I renamed the format for included graphics and not for exported documents,
since otherwise the command line syntax for export would change. This would
require more adaptions for the users, since with the chosen solution the
custom converters are almost always changed correctly in prefs2prefs(),
so that only custom external templates need manual adjustement.