XMLTextParagraphExport: Add a mapping that generates names for those
field marks that don't have a name so validators don't complain.
(regression from 7a1c5e54af)
There is still the problem that we cannot easily guarantee that the
names (generated or from some field param) do not clash with the names
of actual bookmarks...
Change-Id: I9ed65b94b3e8f725db4354222f2565176b32be52
