fdo#47811: RTF import: fix Database field content

1. the Database field master does not have a "Content" property
2. SetFieldContent was called once for every chunk of text in the field
   result, always overwriting the previous value; accumulate the text.

Change-Id: I63cfda19e2e416c52d100d9071796305a5d813c6
(cherry picked from commit e24ae38c8ef233e4b44840058e35959194724743)
Reviewed-on: https://gerrit.libreoffice.org/8442
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>