Created method EPartServiceExt.showPart(String id, PartState partState).
The created method is the fixed version of appropriate method from
EPartService. It search any opened shared part and if allowMultiples ==
false (default) it create placeholder on this part rather creating new.
The differences from original showPart is that it search shared part in
any perspective, the orginal showPart search shared part in active
perspective. I can not say is this bug or general contract of this
method...