1. Simplify handling of mark vector by implementing backward-bias positions over standard positions.
2. Implement positions sharing for positions not affected by text removal.
Fixed failing test (some more fixing necessary).
It should be fixed now.
One more fix of bad text copy.
Thanks, Honza. I think I'll rather make no more commits today ;-)
Integrated into 'main-golden'
User: Miloslav Metelka <email@example.com>
Log: #196508 - Improve document content of BaseDocument.