Merge revs 26523 and 26557 from the fireworks branch.
- fix ownership policy;
- properly implement SetVertical();
- only map splitter and composite frame when adding a 
  new slot instead of calling MapSubwindows().


git-svn-id: http://root.cern.ch/svn/root/trunk@26560 27541ba8-7e3a-0410-8455-c3a389f83636