[AOO 3.4.1 candidate] Support replacement when using the regexp features look-ahead /look-behind

[Herbert Duerr]

Balancing the benefit vs. risk of a change is a non-trivial question 
especially for a micro-release as AOO 3.4.1.

https://issues.apache.org/ooo/show_bug.cgi?id=118925 is such a candidate 
for that release: Writer should keep up with the features of the regexp 
engine that is much more powerful since it has been upgraded in AOO 
3.4.0. Otherwise the asymmetry between the Find and the Replacement 
functionality would be too confusing.

Herbert