Improve behavior of TSessionViewer:
- automatically update the list of existing sessions on the server
  at connection time
- fix the bug #16882, Proof GUI startup: wrong defaults
  (http://savannah.cern.ch/bugs/?func=detailitem&item_id=16882)


git-svn-id: http://root.cern.ch/svn/root/trunk@16320 27541ba8-7e3a-0410-8455-c3a389f83636