In some RTF and DOCX files, field type may be prepended with
backslash. It seems not strictly standard-conformant (at least
not to [MS-RTF]). Still, MS Word reads these fields OK. This patch
allows LO to read them, too.

Change-Id: I1f6bbdd9d57adbea8882d078761dd97298ca03b3
Reviewed-on: https://gerrit.libreoffice.org/20420Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>