fdo#67742 Avoid autocorr of "--" before "-"

LibO 4.1 now uses '-' as word separator for AutoCorrection
Then the sequence "---" is detected as word "--" and autocorrect
as "–". This avoids autocorrection of "---", as word or as border
This commit detects the sequence "---" and report autocorrection.
Modified for LibO 4.1 (OUString => String)

Change-Id: I1342c2ff83dd42683e683b3bb27280d61179b9a2
Reviewed-on: https://gerrit.libreoffice.org/6747
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>