The DOC/ODT import can call SwDoc::SetAllUniqueFlyNames() at the end of
the process to assign unique names to fly frames which lack a name.

Add a similar (but much simpler) feature to the domain mapper to avoid
empty image names in the DOCX/RTF import result, so it's easier to click
on the items in Writer's navigator.

Change-Id: I432fc741f8d75d735e1dfe88daba50ba0797042d
Reviewed-on: https://gerrit.libreoffice.org/25705Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>