libmwaw: avoid 'MWAWDocument::isFileFormatSupported()' when opening .doc files

There is only a single libmwaw switch to enable both "noopt + symbols"
and "debug messages", and we enable that switch in debug builds.
Backport upstream patch to avoid a debug message during type detection.

Change-Id: I53440ceed90529e3db7c9d91c788c7a3b2723eb1
Remove unused default attribute

Change-Id: I66e78b7815173ec9181b9449252da740c10f09ec
RTF import: fix handling of RTF_SHPFBLWTXT

Commit 6cac123a8de8357cf11d9b5f818233889d729939 (fdo#45183 import
RTF_SHPFBLWTXT, 2012-12-27) fixed this already once in the past -- fix
it again, this time with a testcase.

Change-Id: I0cbbfb1ba4eef42b2ee32f6f77065afaad3ddc1b