writerfilter: add a factory for DomainMapper

Ideally the XFilter implementations should only know that DomainMapper
implements Stream, nothing more. Add a factory and use it in RtfFilter.
When ImportFilter will do the same, then the DomainMapper class
definition can be an implementation detail.

Change-Id: If19cf23e61c2f78189d834261d57c569b9173b12
writerfilter: let ImportFilter only assume that DomainMapper is a Stream

Change-Id: I2e0db194357657df81d8cd94f42cdfbd2c3a2664